templates: replace empty name and email strings with placeholders

New placeholders say "(no name availalbe)" and "(no email available)",
because empty strings aren't _necessarily_ a configuration issue.

cli/src/template_builder.rs

@@ -382,20 +382,32 @@ fn build_signature_method<'a, L: TemplateLanguage<'a>>(
         "name" => {
             template_parser::expect_no_arguments(function)?;
             language.wrap_string(TemplateFunction::new(self_property, |signature| {
+                if !signature.name.is_empty() {
                     signature.name
+                } else {
+                    "(no name available)".to_string()
+                }
             }))
         }
         "email" => {
             template_parser::expect_no_arguments(function)?;
             language.wrap_string(TemplateFunction::new(self_property, |signature| {
+                if !signature.email.is_empty() {
                     signature.email
+                } else {
+                    "(no email available)".to_string()
+                }
             }))
         }
         "username" => {
             template_parser::expect_no_arguments(function)?;
             language.wrap_string(TemplateFunction::new(self_property, |signature| {
+                if !signature.email.is_empty() {
                     let (username, _) = text_util::split_email(&signature.email);
                     username.to_owned()
+                } else {
+                    "(no username available)".to_string()
+                }
             }))
         }
         "timestamp" => {

cli/src/templater.rs

@@ -57,9 +57,17 @@ impl<C, T: Template<C> + ?Sized> Template<C> for Box<T> {
 
 impl Template<()> for Signature {
     fn format(&self, _: &(), formatter: &mut dyn Formatter) -> io::Result<()> {
+        if !self.name.is_empty() {
             write!(formatter.labeled("name"), "{}", self.name)?;
+        } else {
+            write!(formatter.labeled("name"), "(no name available)")?;
+        }
         write!(formatter, " <")?;
+        if !self.email.is_empty() {
             write!(formatter.labeled("email"), "{}", self.email)?;
+        } else {
+            write!(formatter.labeled("email"), "(no email available)")?;
+        }
         write!(formatter, ">")?;
         Ok(())
     }

cli/tests/test_commit_template.rs

@@ -187,8 +187,8 @@ fn test_log_builtin_templates() {
     │
     ◉  Commit ID: 0000000000000000000000000000000000000000
        Change ID: zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
-       Author:  <> (1970-01-01 00:00:00.000 +00:00)
+       Author: (no name available) <(no email available)> (1970-01-01 00:00:00.000 +00:00)
-       Committer:  <> (1970-01-01 00:00:00.000 +00:00)
+       Committer: (no name available) <(no email available)> (1970-01-01 00:00:00.000 +00:00)
 
            (no description set)
 

cli/tests/test_templater.rs

@@ -418,6 +418,20 @@ fn test_templater_signature() {
     insta::assert_snapshot!(render(r#"author"#), @"Test User <x@y>");
     insta::assert_snapshot!(render(r#"author.email()"#), @"x@y");
     insta::assert_snapshot!(render(r#"author.username()"#), @"x");
+
+    test_env.jj_cmd_ok(&repo_path, &["--config-toml=user.name=''", "new"]);
+
+    insta::assert_snapshot!(render(r#"author"#), @"(no name available) <test.user@example.com>");
+    insta::assert_snapshot!(render(r#"author.name()"#), @"(no name available)");
+    insta::assert_snapshot!(render(r#"author.email()"#), @"test.user@example.com");
+    insta::assert_snapshot!(render(r#"author.username()"#), @"test.user");
+
+    test_env.jj_cmd_ok(&repo_path, &["--config-toml=user.email=''", "new"]);
+
+    insta::assert_snapshot!(render(r#"author"#), @"Test User <(no email available)>");
+    insta::assert_snapshot!(render(r#"author.name()"#), @"Test User");
+    insta::assert_snapshot!(render(r#"author.email()"#), @"(no email available)");
+    insta::assert_snapshot!(render(r#"author.username()"#), @"(no username available)");
 }
 
 #[test]
@@ -601,7 +615,7 @@ fn test_templater_concat_function() {
     let render = |template| get_colored_template_output(&test_env, &repo_path, "@-", template);
 
     insta::assert_snapshot!(render(r#"concat()"#), @"");
-    insta::assert_snapshot!(render(r#"concat(author, empty)"#), @" <>true");
+    insta::assert_snapshot!(render(r#"concat(author, empty)"#), @"(no name available) <(no email available)>true");
     insta::assert_snapshot!(
         render(r#"concat(label("error", ""), label("warning", "a"), "b")"#),
         @"ab");
@@ -651,11 +665,11 @@ fn test_templater_separate_function() {
 
     // Separate keywords
     insta::assert_snapshot!(
-        render(r#"separate(" ", author, description, empty)"#), @" <> true");
+        render(r#"separate(" ", author, description, empty)"#), @"(no name available) <(no email available)> true");
 
     // Keyword as separator
     insta::assert_snapshot!(
-        render(r#"separate(author, "X", "Y", "Z")"#), @"X <>Y <>Z");
+        render(r#"separate(author, "X", "Y", "Z")"#), @"X(no name available) <(no email available)>Y(no name available) <(no email available)>Z");
 }
 
 #[test]