plist-sort: fix empty PLIST_SUB substitutions

[LaurentChardon]

Fix handling of empty PLIST_SUB variables.

Variables defined as empty were incorrectly collapsed from "%%VAR%%" to "VAR".

[ppekala]

Hello, thanks for submitting this PR.
Unfortunately it does not solve whole problem you are describing.
It works for one case:

%%VAR%%/some/file

but will fail for more complicated lines, like for example:

%%PORTDOCS%%%%VAR%%/some/file.

I would like to fix all cases in one go and need some time to debug further.
Can you provide some example ports that fail in your testing?

[LaurentChardon]

Hi, thanks for looking into my patch.
I have made this patch to fix the very common situation where a variable is either "" or "@comment ", to handle port OPTIONS. There are hundreds of ports in this situation (442 if my portgrep command works as intended). It's really the only case that this patch intended to fix. Because of the "@comment" keyword, this substitution has to be the first one in order to work.
The particular plist file that I was sorting is the one from emulators/mame.
Thanks!