Followup to #1642, closes #1859

This change adds back the ability to throw out both empty strings
and null values with the not equals statement like `[attr] != ''`

Note: this does not mean null == '' because the needed compatibility
fix only addresses the "not equals` operator to retain the historical
usage of not matching nulls.
Merge pull request #1880 from mapnik/null-empty-string-not-equality

Make [attr] != '' match nulls: Followup to #1642, closes #1859