[Proto] Prevent emit when no type declarations are present

Author: Linwenxuan04

Lagrange.Proto.Generator/ProtoSourceGenerator.cs

@@ -27,7 +27,9 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
     private static void Emit(SourceProductionContext context, Parser parser)
     {
         foreach (var diagnostic in parser.Diagnostics) context.ReportDiagnostic(diagnostic);
-        
+
+        if (parser.TypeDeclarations.Count == 0) return;
+
         var emitter = new Emitter(parser);
         emitter.Emit(context);
     }