lwgeom_extent_to_gml3 segfaults when passed an LWGEOM with no BBOX cache

[strk]

When passing an LWGEOM with no bounding box to lwgeom_extent_to_gml3, it segfaults.

I haven't tried with lwgeom_extent_to_gml2. Beside, I'm also not sure why these functions take an LWGEOM rather than just a GBOX.

[strk]

See also #1322 for a case in which this showed up.

[strk]

What should this function return for an empty geometry ? For comparison, ST_AsGML('LINESTRING EMPTY') returns:

 <gml:LineString><gml:coordinates></gml:coordinates></gml:LineString>

.. which I'm not sure is correct either ..

[strk]

Fixed by r8333, which includes comprensive regress testing. The EMPTY case may be subject to change.

Note that the commit also fixes support for srsDimension option (don't remember how to specify that option at the SQL level but should be written in the manual page, the tests I added were at the liblwgeom level).

[strk]

r8334 reworks the empty box GML to be empty tags <Box/> and <Envelope/>