client API: allow calling mpv_terminate_destroy(NULL)

This is an oversight and a bug.

CC: @mpv-player/stable

player/client.c

@@ -330,6 +330,9 @@ static void get_thread(void *ptr)
 
 void mpv_terminate_destroy(mpv_handle *ctx)
 {
+    if (!ctx)
+        return;
+
     mpv_command(ctx, (const char*[]){"quit", NULL});
 
     if (!ctx->owner) {