How to check bucket size condition and change email action in watcher?

Hello everyone,

I have a watcher:

  "input": {
    "search": {
      "request": {
        "search_type": "query_then_fetch",
        "indices": [
        "body": {
          "query": {"match_all": {}},
          "aggs": {
            "group_a": {},
            "group_b": {},
            "group_c": {}

With the watcher action, how can I check whether the groups have data or not? For example:
If ctx.payload.aggregations.group_a.buckets.size() = 0 --> Email:

Group B:
LIST GROUP A {{key}} and {{doc_count}}
Group C:
LIST GROUP C {{key}} and {{doc_count}}

Similar to other groups, if the group has no data, it will be removed from the warning email