Spatial: Translate EnvelopeCombiner.CombineAsGeometry aggregate function

[roji]

@airbreather graciously added EnvelopeCombiner.Combine for us in NetTopologySuite/NetTopologySuite#610, so once NTS is next released we can add an aggregate method translation for that as a follow-up to #13278:

.NET	SQL Server	SpatiaLite	PostGIS
EnvelopeCombiner.CombineAsGeometry(geoms)	EnvelopeAggregate(@​geoms)	Extent(@​geoms)	ST_Extent(@​geoms)

[douglasg14b]

Are the code example of how this works? For instance using EnvelopeCombiner.CombineAsGeometry ?

Just trying to get a SELECT st_extent(position) from my_table is seemingly harder than I thought it would be using this....

await _context.Screens
            .GroupBy(x => 1)
            .Select(g => EnvelopeCombiner.CombineAsGeometry(g.Select(x => x.Position)))
            .FirstOrDefaultAsync();

results in:

SELECT ST_Extent(t.position)::geometry
      FROM (
          SELECT s.position, 1 AS "Key"
          FROM screens AS s
      ) AS t
      GROUP BY t."Key"
      LIMIT 1

Can this be used without a group by? What is the intended behavior?