QWasmEventTranslator: return Key_unknown, not 0x00, when a key isn't found

This matches what translateDeadKey() returns for unhandled keys, and
processKeyboard() checks for when replacing qtKey with the result of
translateDeadKey().

Change-Id: I1500576b7b31047a7a35633a15cd6975b77d842d
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/plugins/platforms/wasm/qwasmeventtranslator.cpp

@@ -824,7 +824,7 @@ static Qt::Key find_impl(const KeyMapping *first, const KeyMapping *last, Qt::Ke
             return first->to;
         ++first;
     }
-    return {};
+    return Qt::Key_unknown;
 }
 
 template <size_t N>