Converts a multivalued expression into a single valued column containing the first value. This is most useful when reading from a function that emits multivalued columns in a known order like <<esql-split>>.
The order that <<esql-multivalued-fields, multivalued fields>> are read from
underlying storage is not guaranteed. It is *frequently* ascending, but don't
rely on that. If you need the minimum value use <<esql-mv_min>> instead of
`MV_FIRST`. `MV_MIN` has optimizations for sorted values so there isn't a
