Fix return value in GraphBuilder.visitVariableDeclaration.

This method should return `null`, since it doesn't visit an expression.

Change-Id: I018241c0d2e9b9f00bfe293352cf2707551c81e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/104881
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/analysis_server/lib/src/nullability/graph_builder.dart

@@ -472,8 +472,9 @@ class GraphBuilder extends GeneralizingAstVisitor<DecoratedType> {
     if (initializer == null) {
       throw UnimplementedError('TODO(paulberry)');
     } else {
-      return _handleAssignment(destinationType, initializer);
+      _handleAssignment(destinationType, initializer);
     }
+    return null;
   }
 
   /// Creates the necessary constraint(s) for an assignment from [sourceType] to