Add unit test for filter printer handling 0 values
Allow filters equal to 0 in filter printer

Prior to this patch, if you were filtering on the value 0, the default
filter printer would not display it, so you could end up with, for example,
", 0.05" in the controls when it should actually say "0, 0.05". This patch
checks explicitly for undefined and null filters.
Merge pull request #715