Merge/squash commit templates: avoid breaking Git trailers

What does this MR do and why?

Solves #349622 (closed)

In the Merge/Squash commit message templates we are removing variables with empty values along with the blank lines before it. It was made to allow recreating default merge commit template:

Merge branch '%{source_branch}' into '%{target_branch}'

%{title}

%{issues}

See merge request %{reference}

and if the issues are empty, we would remove the preceding new lines:

Merge branch '%{source_branch}' into '%{target_branch}'

%{title}

See merge request %{reference}

However, recently we've added variables that form Git trailers by themselves - in !76042 (merged) we've added %{co_authored_by}, and in !75639 (merged) we've added %{approved_by}.

This blank line removal can also remove the blank line that is required to form valid Git trailers:

%{title}

%{co_authored_by}
MR: %{url}
Merged-By: %{merged_by}

and if the %{co_authored_by} is empty it becomes:

%{title}
MR: %{url}
Merged-By: %{merged_by}

This MR fixes it, so that instead it becomes:

%{title}

MR: %{url}
Merged-By: %{merged_by}

This change simply removes the preceding blank line only if the variable is also followed by a blank line. So we can still recreate the default templates, but we no longer remove blank lines that can break formatting of a message.

How to set up and validate locally

Follow http://localhost:3005/ee/user/project/merge_requests/commit_templates.html to fill in merge/squash templates. I've used the following template to validate it:

%{title}

%{co_authored_by}
MR: %{url}
Merged-By: %{merged_by}

without any co-authors in a MR.

And also the default one:

Merge branch '%{source_branch}' into '%{target_branch}'

%{title}

%{issues}

See merge request %{reference}

to make sure there's no regression.

MR acceptance checklist

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