Remove whitespaces of whitespace-only files (#3348)

Currently, empty and whitespace-only (with or without newlines) are
not modified. In some discussions (issues and pull requests) consensus
was to reformat whitespace-only files to empty or single-character
files, preserving line endings when possible. With that said, this
commit introduces the following behaviors:

* Empty files are left as is
* Whitespace-only files (no newline) reformat into empty files
* Whitespace-only files (1 or more newlines) reformat into a single
newline character

To implement these changes, we moved the initial check at
`format_file_contents` that raises `NothingChanged` if the source
(with no whitespaces) is an empty string. In the case of *.ipynb
files, `format_ipynb_string` checks a similar condition and removed
whitespaces. In the case of Python files, `format_str_once` includes a
check on the output that returns the correct newline character if
possible or an empty string otherwise.

Signed-off-by: Antonio Ossa Guerra <aaossa@uc.cl>