templater: extend filter() to accept template expression for emptiness test This utilizes the pass-by-name nature of template arguments.

templater: introduce filter() function to remove empty items from list The primary use case is to filter out "tip" from a list of tags.

templater: fix truth testing of integer 0 taken from a list/dict Broken at f9c426385853. bool(python_value) shouldn't be used here since an integer 0 has to be truthy for backward compatibility.