Joining an empty list should return an empty list

[rmunn]

Fixes #4852.

Note that this changes behavior introduced in TiddlyWiki 5.1.22 by #4396. Specifically, #4396 made join return [""] (a list containing a single empty string) if passed an empty input list, while this PR makes join return [] (an empty list) if passed an empty input list. Before #4396, join would return [null] (a list containing a single null) on empty input, which was causing Red Screen of Death errors. While returning [""] does stop the crashes, I believe that it's not what users would expect, and users would expect join to return an empty list when given empty input.

I would also like code-style comments: I've written if(result.length === 0) return []; as a single line here. Should I instead write this with braces? It seems short enough to merit a one-liner, but some people don't like that.

[Jermolene]

Thanks @rmunn that makes sense.

I would also like code-style comments: I've written if(result.length === 0) return []; as a single line here. Should I instead write this with braces? It seems short enough to merit a one-liner, but some people don't like that.

Our house style is to always use braces.

[rmunn]

@Jermolene Okay, I've rewritten this PR to use braces instead. And while I was at it, I rebased it onto the current state of the master branch.

[rmunn]

@Jermolene Anything else you want me to change here, or is this ready to merge?

[Jermolene]

Thanks @rmunn apologies for the delay.