Ignore empty completion input

[jimczi]

This change makes sure that an empty completion input does not throw an IAE when indexing.
Instead the input is simply ignored.

Closes #23121

[mayya-sharipova]

+1 LGTM

I was wondering if this input.length ==0 after the input has already been analyzed?

[jimczi]

cc @elastic/es-search-aggs

[jimczi]

This pr is stalled because we need to decide if we want to ignore empty fields silently. There are some concerns that this change could hide bugs at ingestion if we simply ignore empty strings. I added the discuss label in order to discuss the different options.

[jimczi]

Now that we have an _ignored field we can add the completion field to the list in order to indicate that the suggestion was ignored:
https://www.elastic.co/guide/en/elasticsearch/reference/master/mapping-ignored-field.html
I opened a new pr #30713 to implement this idea so I am closing this one.