[USER32] InternalGetWindowText: Write to caller's buffer only when one was provided. CORE-13614

win32ss/user/user32/windows/window.c

@@ -1799,7 +1799,7 @@ int WINAPI
 InternalGetWindowText(HWND hWnd, LPWSTR lpString, int nMaxCount)
 {
     INT Ret = NtUserInternalGetWindowText(hWnd, lpString, nMaxCount);
-    if (Ret == 0)
+    if (Ret == 0 && lpString)
         *lpString = L'\0';
     return Ret;
 }