When newline_style is set to Windows, an empty line inside of a macro
results in `\r` being passed to the `fold()` in `MacroBranch::rewrite()`.
`\r` is technically not an empty string, so we try to indent it, leaving
trailing whitespaces behind, even though that was not intended
(as far as I can see).
This commit replaces the `!l.is_empty()` check with calling
`is_empty_line()`, since trying to indent any whitespace-only string
will probably result in problematic trailing whitespaces.
Fixes: #2810
