Explicitly set default values for fields of struct JSRuntimeFlags (#54189)

Summary:
Pull Request resolved: #54189

Explicitly set default values for fields of struct JSRuntimeFlags

changelog: [internal] internal

Reviewed By: cortinico

Differential Revision: D84952242

fbshipit-source-id: 41de7eb791c9880c83573d670f4a777bf0a753d9

Author: mdvacca

packages/react-native/ReactCommon/react/runtime/ReactInstance.h

@@ -40,7 +40,7 @@ class ReactInstance final : private jsinspector_modern::InstanceTargetDelegate {
 
   struct JSRuntimeFlags {
     bool isProfiling = false;
-    const std::string runtimeDiagnosticFlags;
+    const std::string runtimeDiagnosticFlags = {};
   };
 
   void initializeRuntime(