ESQL: Don't build empty `ValuesSourceReaderOperator`s

[nik9000]

Description

In ESQL ValuesSourceReaderOperator is the rather anachronistic name of the Operator that reads doc values and stored fields from the lucene index. I'm working on it now to split pages in #130573. I've found out that we sometimes create a ValuesSourceReaderOperator when telling it to load 0 fields. This is redundant. And adds complexity to the code:

                // TODO this is needed when we load 0 fields. Can we just skip building the Operator at all in that case?

Let's fix it!

We could assert in the factory that the list of fields we're loading is non-empty.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[nik9000]

Looks like I'm going to need to do this as part of #130573. It's just too hacky to try and work around it. Checking.