wasm: fix null-argument warning

That's "null passed to a callee that requires a non-null argument".

Pass EMSCRIPTEN_EVENT_TARGET_WINDOW instead of NULL to
emscripten_set_wheel_callback. The function is never called
here, so it does not matter which non-null value it
gets.

Pick-to: 6.6
Change-Id: I10187a222513820158a938bf05cde60f489956f1
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/corelib/platform/wasm/qstdweb.cpp

@@ -34,7 +34,7 @@ static void usePotentialyUnusedSymbols()
     // called at runtime.
     volatile bool doIt = false;
     if (doIt)
-        emscripten_set_wheel_callback("", 0, 0, NULL);
+        emscripten_set_wheel_callback(EMSCRIPTEN_EVENT_TARGET_WINDOW, 0, 0, NULL);
 }
 
 Q_CONSTRUCTOR_FUNCTION(usePotentialyUnusedSymbols)