Manually add t/ackrc-empty to MANIFEST

Ack.pm checks $ENV{ACKRC} but if the file doesn't exist
it moves on to the next candidate ($HOME/.ackrc).

I noticed that you merged my change, but 1.96 still errored on my machine.
Sorry, I didn't realize MANIFEST was managed manually.

Just in case you didn't realize, there are quite a few other files that are not included there, notably a few other files in t/.
When I do `make manifest` it prints the following... i suspect you wouldn't want all these changes,
but there may be something else that has been overlooked.

```
Added to MANIFEST: ack-help-dirs.txt
Added to MANIFEST: ack-names.markdown
Added to MANIFEST: api.pod
Added to MANIFEST: btg/ack-help-types.txt
Added to MANIFEST: btg/ack-help.txt
Added to MANIFEST: btg/ack-lists.php
Added to MANIFEST: btg/ack-standalone
Added to MANIFEST: btg/Changes
Added to MANIFEST: btg/index.php
Added to MANIFEST: etc/ack.zsh_completion
Added to MANIFEST: perlcriticrc
Added to MANIFEST: perltidyrc
Added to MANIFEST: t/ack-show-types.t
Added to MANIFEST: t/ackrc.t
Removed from MANIFEST: t/swamp/#emacs-workfile.pl#
Removed from MANIFEST: t/swamp/blib/ignore.pir
Removed from MANIFEST: t/swamp/blib/ignore.pm
Removed from MANIFEST: t/swamp/blib/ignore.pod
Removed from MANIFEST: t/swamp/groceries/another_subdir/CVS/fruit
Removed from MANIFEST: t/swamp/groceries/another_subdir/CVS/junk
Removed from MANIFEST: t/swamp/groceries/another_subdir/CVS/meat
Removed from MANIFEST: t/swamp/groceries/another_subdir/RCS/fruit
Removed from MANIFEST: t/swamp/groceries/another_subdir/RCS/junk
Removed from MANIFEST: t/swamp/groceries/another_subdir/RCS/meat
Removed from MANIFEST: t/swamp/groceries/CVS/fruit
Removed from MANIFEST: t/swamp/groceries/CVS/junk
Removed from MANIFEST: t/swamp/groceries/CVS/meat
Removed from MANIFEST: t/swamp/groceries/RCS/fruit
Removed from MANIFEST: t/swamp/groceries/RCS/junk
Removed from MANIFEST: t/swamp/groceries/RCS/meat
Removed from MANIFEST: t/swamp/Makefile
Removed from MANIFEST: t/swamp/not-an-#emacs-workfile#
Removed from MANIFEST: t/swamp/options.pl.bak
Added to MANIFEST: tags
Added to MANIFEST: Tar.pm
Added to MANIFEST: ttest
```