templater: don't panic on PropertyPlaceholder unset error

The idea is that, if .extract() succeeded in static context, it means the
property can be evaluated as constant. This will potentially eliminate
expect_string_literal_with(), though I'm not too sure if it's a good idea.
If needed, maybe we can extend the idea to suppress type/name resolution errors
by "if(some_static_config_knob, x, y)".

cli/src/templater.rs

@@ -620,12 +620,11 @@ impl<O: Clone> TemplateProperty for PropertyPlaceholder<O> {
     type Output = O;
 
     fn extract(&self) -> Result<Self::Output, TemplatePropertyError> {
-        Ok(self
+        if let Some(value) = self.value.borrow().as_ref() {
-            .value
+            Ok(value.clone())
-            .borrow()
+        } else {
-            .as_ref()
+            Err(TemplatePropertyError("Placeholder value is not set".into()))
-            .expect("placeholder value must be set before evaluating template")
+        }
-            .clone())
     }
 }