Avoid index errors when input file is empty

[m-akinc]

When we try to add suppressions in empty files, it results in index out of range errors. Really, it's a result of a linter error being reported on a non-existent line (i.e. line 1 of empty file). The _add_noqa_to_line function takes in the array of lines and modifies the contents, so the easiest way to avoid this problem is to populate the lines array with a single blank line when the file is empty. Also, the multiline checker has a similar problem, but we can avoid that easily by checking that the error.line is within range before calling multiline_checker.in_multiline_string.

[mshafer-NI]

Can we add these cases to the system tests?
https://github.com/ni/python-styleguide/tree/main/tests/test_cli/acknowledge_existing_errors_test_cases__snapshots

[m-akinc]

Can we add these cases to the system tests?
https://github.com/ni/python-styleguide/tree/main/tests/test_cli/acknowledge_existing_errors_test_cases__snapshots

Added test for empty file

[mshafer-NI]

Looks good - thanks for adding the test