diff: fix filtering of merge commits under --remerge-diff

diff: fix filtering of merge commits under --remerge-diff

Commit 95433eeed9 ("diff: add ability to insert additional headers for
paths", 2022-02-02) introduced the possibility of additional headers.
Because there could be conflicts with no content differences (e.g. a
modify/delete conflict resolved in favor of taking the modified file
as-is), that commit also modified the diff_queue_is_empty() and
diff_flush_patch() logic to ensure these headers were included even if
there was no associated content diff.

However, the added logic was a bit inconsistent between these two
functions.  diff_queue_is_empty() overlooked the fact that the
additional headers strmap could be non-NULL and empty, which would cause
it to display commits that should have been filtered out.

Fix the diff_queue_is_empty() logic to also account for
additional_path_headers being empty.

Reported-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>