ST_GetFaceGeometry exception on invalid edge, null on unexistent face

[strk]

The SQL/MM document says that an exception should be raised IF the face argument references an unknown face. Doesn't say anything about faces bound by no ring (would be due to invalid topology).

The current implementation results in an exception in the latter case (no ring) but not in the former (unknown face).

In turn this doesn't help validating a topology in that the exception on invalid topology makes the whole validateTopology function bail out.

[strk]

This is actually ST_BuildArea's fault, whereas passing 'LINESTRING(0 0, 10 10)' as input returns NULL while 'LINESTRING(0 0)' as input returns an exception (cause the input is invalid...).

[strk]

r9184 fixes the exception by making even single-point edges convertible to GEOS. The null-on-unexistent face I'll pretend I didnt see (would only make things slower)