Fix difficultycolor for units with a level of -1.

[gizzmo]

Since the inception of the difficultycolor tag, (yes i checked the git history) it has suffered from the issue of showing the grey trivial color for bosses, as their UnitLevel will return -1.

This change better matches oUF's difficulty tag.

Screenshot:
Top "??" uses [difficulty][level],
while lower uses [difficultycolor][level]

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Seeing as this is how the tag has always displayed, and no one really noticed (myself included). I'd assume everyone is used to it being that color. Thoughts?

[Repooc]

I personally don't agree to change the tag as it has been like this since it was made and assumed it was on purpose, I don't see the need to change it as a user can just use the ouf's version at that point. Others may agree with you, so not closing it without other input