Currently, empty and whitespace-only (with or without newlines) are
not modified. In some discussions (issues and pull requests) consesus
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>

This commit introduces two tests and modifies other two tests. The
introduced tests verify the expected behaviour on files containing a
single newline character via 1) `format_str` and 2)
`format_file_in_place`. In the other hand, the first modified test
(`test_format_file_contents`) verify that `NothingChanged` is raised
on files containing only a single newline, and that whitespace-only
files are properly formatted to a single newline character. The second
modified test (`test_reformat_one_with_stdin_empty`) validates the
expected behavior when the input is passed via stdin.

Before the fix introduced in the previous commit, these tests (and a
couple of others) failed on cases covering whitespace-only files. Now,
these tests are passed for all cases.

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

Add entry about new behavior on whitespace-only files: removing
whitespace characters and return a single newline (if present) or an
empty file

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

The new criteria to reformat empty and whitespace-only files should go
into the preview style

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

Adding comments on a non-intuitive behavior ar `_format_str_once`, and
improving the `test_one_empty_line` test readability by using a
pre-defined function that implements the same assertions

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