analyzer_plugin: Add missing return in nullably-typed function

Bug: https://github.com/dart-lang/sdk/issues/46656
Change-Id: I8a37d4b08ea678b7d021f173448d6770b04f1d34
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/222384
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

pkg/analyzer_plugin/lib/src/utilities/completion/completion_target.dart

@@ -262,6 +262,7 @@ class CompletionTarget {
         return node.prefix;
       }
     }
+    return null;
   }
 
   /// If the target is an argument in an argument list, and the invocation is