Fix QTabBar crash with offscreen platform plugin

The offscreen implementation does not have a QPlatformNativeInterface
implementation.

Pick-to: 5.15
Change-Id: Ife4f296f52c307a2fab90de2cdc1ef0cc7796385
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/widgets/qtabbar.cpp

@@ -139,6 +139,8 @@ void QTabBarPrivate::updateMacBorderMetrics()
     }
 
     QPlatformNativeInterface *nativeInterface = QGuiApplication::platformNativeInterface();
+    if (!nativeInterface)
+        return;
     quintptr identifier = reinterpret_cast<quintptr>(q);
 
     // Set geometry