empty text labels get rendered as zero (0)

[jburgess777]

When running the SVN mapnik tree I get lots of 0 for text labels. This occurs of a road has no label. This started happening since r657 removed NUL columns from the postgis result set.

The problem occurs because the text symbolizer accesses 'feature[sym.get_name()]' which will return a value(0) if the name does not exist in the feature properties.

I have a tentative fix which I'll attach.

[jburgess777]

The patch looks massive but is mostly white space changes (do 'svn diff -x -w' on the result to see something more reasonable)

[jburgess777]

example tile shoiwing 0's

[tomh]

Patch committed as r692.

[tomh]

Talking with Jon on IRC we think a better solution may be an explicit null type in the boost::variant used by mapnik::value...

[tomh]

Original fix reverted and new improved fix committed as r693. This fixes both the problem described here and the problem with over matching by rules which do not equals comparisons on attributes which do not exist.