[PostGIS] #5835: The variants of ST_Union gives results that are inconsistent with each other

[PostGIS]

#5835: The variants of ST_Union gives results that are inconsistent with each
other
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  Reporter:  nbvfgh   |      Owner:  pramsey
      Type:  defect   |     Status:  new
  Priority:  high     |  Milestone:  PostGIS 3.6.0
 Component:  postgis  |    Version:  3.5.x
Resolution:           |   Keywords:
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Comment (by mdavis):

 One bug is that the overlay operations should not return collections
 containing empty elements.

 Apart from that, the inconsistency is probably due to two things:
 * the implementation of `ST_Union(A, B)` may have a bug in handling
 `GeometryCollection`s
 * the implementation of `ST_Union(C)` likely strips zero-length
 `LineString`s from the output.  This is a GEOS bug documented here:
 https://github.com/libgeos/geos/issues/1136
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/5835#comment:2>
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