winmm: mixerOpen should return success with NULL dwCallback and CALLBACK_WINDOW flag.

dlls/winmm/tests/mixer.c

@@ -1035,7 +1035,6 @@ static void test_mixerOpen()
 
         /* Test mixerOpen with a NULL dwCallback and CALLBACK_WINDOW flag. */
         rc = mixerOpen(&mix, d, 0, 0, CALLBACK_WINDOW);
-        todo_wine
         ok(rc == MMSYSERR_NOERROR,
            "mixerOpen: MMSYSERR_NOERROR expected, got %s\n",
            mmsys_error(rc));

dlls/winmm/winmm.c

@@ -355,7 +355,7 @@ UINT  MIXER_Open(LPHMIXER lphMix, UINT uDeviceID, DWORD_PTR dwCallback,
 
     case CALLBACK_WINDOW:
         mod.dwInstance = dwCallback;
-        if (!IsWindow((HWND)dwCallback))
+        if (dwCallback && !IsWindow((HWND)dwCallback))
             return MMSYSERR_INVALPARAM;
         break;
     }