Change QT_FATAL_WARNINGS behavior to require a non-empty value

This allows easy unsetting of the variable in a shell like:
  QT_FATAL_WARNINGS= progname

Change-Id: Ie9cfb6ebfd4931de1c90af68bfeeae1e9f3d4b9d
Reviewed-by: Kai Koehne <kai.koehne@digia.com>

src/corelib/global/qlogging.cpp

@@ -73,7 +73,7 @@ static bool isFatal(QtMsgType msgType)
         return true;
 
     if (msgType == QtWarningMsg) {
-        static bool fatalWarnings = qEnvironmentVariableIsSet("QT_FATAL_WARNINGS");
+        static bool fatalWarnings = !qEnvironmentVariableIsEmpty("QT_FATAL_WARNINGS");
         return fatalWarnings;
     }