opt: do not crash in av_set_options_string() if opts == NULL

Add missing NULL check, and update documentation accordingly.

libavutil/opt.c

@@ -521,6 +521,8 @@ int av_set_options_string(void *ctx, const char *opts,
 {
     int ret, count = 0;
 
+    if (!opts)
+        return 0;
     while (*opts) {
         if ((ret = parse_key_value_pair(ctx, &opts, key_val_sep, pairs_sep)) < 0)
             return ret;