fix(mdns): Fix mdns mdns_lookup_service() to handle empty TXT

the lookup_service API calls _copy_mdns_txt_items(), which tries to
allocate new TXT records, but didn't handle the case with no TXT.
Originally the _copy_mdns_txt_items() called calloc() with zero's which
returned NULL (on espressif toolchain), so it's safe, but we could see
an error message:
E (1170) mdns: Cannot allocate memory (line: 6191, free heap: 281368 bytes)
This commit addresses the empty TXT case and gets rid of the error
message.

components/mdns/mdns.c

@@ -6185,6 +6185,10 @@ static mdns_txt_item_t *_copy_mdns_txt_items(mdns_txt_linked_item_t *items, uint
         ret_index++;
     }
     *txt_count = ret_index;
+    if (ret_index == 0) {   // handle empty TXT
+        *txt_value_len = NULL;
+        return NULL;
+    }
     ret = (mdns_txt_item_t *)calloc(ret_index, sizeof(mdns_txt_item_t));
     *txt_value_len = (uint8_t *)calloc(ret_index, sizeof(uint8_t));
     if (!ret || !(*txt_value_len)) {