Fix Serialize method for Attribute Table to return NULL when no rows or columns

[Sam Gillingham]

Currently gdalinfo shows a "<GDALRasterAttributeTable />" node for each band when there is nothing in the RAT.

The problem is that with the RFC40 changes the HFA driver always returns something from GetDefaultRAT even if it is an empty table - you can now start writing data directly into the table with the returned object.

Attached is a patch to prevent the Serialize() method returning anything when the row and column count is zero

[Sam Gillingham]

Patch to make Serialize return NULL when no rows or columns in RAT

[ilucena]

At revision: 26399

[Kyle Shannon]

Ivan, a couple of tests are failing now and probably need to be updated in autotest/gcore/gdal_api_proxy.py. gdal_api_proxy_(1|2|3) all fail now and all have to do with RAT.

[Even Rouault]

I was tired of the travis bots spamming me about failed builds, so ...r26416 "Fix failed tests after changes done in #5232 and #5119"