If a t tag has no text then the parser throws an error.

For some word processors (cough abiword cough), it is possible for `t` tags to have no content. It would look something like this `<w:t />`.

When we call `parse_t` we call call `self.escape`. If you pass `None` into `self.escape` an error is thrown. If `t.text is None` then early `return ''`