QtQml: Avoid maybe-uninitialized warnings in QJSPrimitiveValue

The compiler can't see that we only access m_string if it has
previously been initialized. However, we can just zero the storage on
construction.

Pick-to: 6.9 6.8 6.5
Fixes: QTBUG-135367
Change-Id: I1b04d81e18063650501bdc24474c9672e999189d
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/qml/jsapi/qjsprimitivevalue.h

@@ -932,10 +932,14 @@ private:
         }
 
         union {
-            bool m_bool = false;
+            bool m_bool;
             int m_int;
             double m_double;
             QString m_string;
+
+            // Dummy value to trigger initialization of the whole storage.
+            // We don't want to see maybe-uninitialized warnings every time we access m_string.
+            std::byte m_zeroInitialize[sizeof(QString)] = {};
         };
 
         Type m_type = Undefined;