Need support for EMPTY geometry forms.

[warmerdam]

We need to add support for the various special forms for empty geometries
and geometry collections in the WKT translator, and it may affect other areas.

eg. 

The only thing we dont support are funny geometries like:

LINESTRING(EMPTY)   {use MULTILINESTRING(EMPTY) instead}
POINT(EMPTY)   {use MULTIPOINT(EMPTY) instead}
POLYGON((EMPTY))   {use MULTIPOLYGON(EMPTY) instead}

Dave Blasby reports the details on format and semantics can be found in the
OGC Simple Features for SQL spec.

[warmerdam]

Implemented support for EMPTY on POINT, LINESTRING, POLYGON, 
GEOMETRYCOLLECTION, MULTIPOINT, MULTILINESTRING, and MULTIPOLYGON. 

Point is a bit broken in that ingesting an empty point and exporting to 
WKT results in POINT(0 0).  All others should have correct results. 

Test script (ogr/ogr_wktempty.py) written to verify operation.