Rewrite empty line handling in LineBasedUnifiedDiffFormatter

The code to replace empty lines with a non-breaking space character became
dead code with the previous change, because there are no empty lines in
the diff any more. All extra newline characters that could form an empty
line are part of the previous paragraph now.

The new code in this patch r-trims paragraphs in the conflict view in a way
that equal numbers of linebreaks at the end of each paragraph get trimmed.
If the number is not identical, the difference remains, and becomes visible
for the user.

Bug: T202402
Change-Id: Ibdaa16ed35fefd0b608da283bb52800f83ddb8ee