tag: fix bug when there are no available tags

[Amorymeltzer]

If there are no available tags (all are already present or none provided) the module would die when trying to untag (like a printer low on cyan). This is incredibly unlikely on enwiki, but I can theoretically imagine other wikis with fewer maintenance templates might run into an issue where all available templates are already transcluded. Reported on my testwiki talk.

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I spent some time delving into other tightenings when the various tag objects were empty or emptied, but it's only partially doable without some major restructuring, and I'm not sure it's even a good idea.

[siddharthvp]

A more elegant long-term solution, I believe, is to make Morebits.quickForm.getElements and HTMLFormElement.prototype.getChecked and the like return an empty array if there is nothing to be returned. This is more intuitive, removes the need to write such special cases, and is the model followed by jQuery. This would not break any existing code (it just renders a if condition useless, in case anyone is using).

[Amorymeltzer]

Yeah — intuitively that makes sense, not sure why null was chosen? I suppose it's strictly more "correct" in some view? I haven't looked at what/where that would require changing