'GEOMETRYCOLLECTION EMPTY' causes 'Unsupported GEOS geometry type' error

[tamas]

Various geometry operations may return 'GEOMETRYCOLLECTION EMPTY' which should be converted to null instead or raising an error.

To fix this we may use this condition in the default case of msGEOSGeometry2Shape:

if (!GEOSisEmpty(g))
        msSetError(MS_GEOSERR, "Unsupported GEOS geometry type (%d).", "msGEOSGeometry2Shape()", type);

[tamas]

Applied in trunk (r11470)