gh-71765: Fix inspect.getsource() on empty file (GH-20809)

* bpo-27578: Fix inspect.getsource() on empty file

For modules from empty files, `inspect.getsource()` now
returns an empty string, and `inspect.getsourcelines()` returns
a list of one empty string, fixing the expected invariant.

As indicated by `exec('')`, empty strings are valid Python
source code.

Co-authored-by: Oleg Iarygin <oleg@arhadthedev.net>