MAINT: Refactor Python engine empty line funcs
The Python engine's `_empty` and `_check_empty` methods were uninformative and undocumented.

This commit renames them to `_is_line_empty` and `_remove_empty_lines` respectively and provides appropriate documentation.

xref <a href="https://github.com/pandas-dev/pandas/pull/15925#discussion_r110377233">#15925 (comment)</a>


