fdo#74573: Write test for pasting with empty cells skipped.

Change-Id: I8ede86b248a9b3a17d077442537e2ec37034f597
fdo#74573: Skip deletion of destination area when 'skip empty' is on.

Also, adjust handling of mix document aka paste functions with this
change.  When using paste function (add, subtract, etc), the behavior
between the 'skip empty' flag on and off makes no difference.  Let's
set the flag to off when paste function is used.

Change-Id: I67724ba923c9260b2c14464e4123b8445712dbaf