syntax: Quote should probably return "" instead of empty for an empty string

[andreynering]

Originally reported at go-task/task#609.

If you quote an empty string the returned value won't work in a concatenated Bash script: == 'foo' vs. the desired "" == "foo".

I think it'd make sense to fix this here, because more people could use the returned value inside Bash scripts. But if you don't think so, I gonna do this fix on the Task side.

[mvdan]

This actually got caught by the fuzzer after I improved it a little, and is already fixed on master; see 92eab20.

I'm preparing for a v3.4.1 release soon, but I actually forgot to cherry-pick that into https://github.com/mvdan/sh/tree/v3.4 - oops. Fixed now. I plan on doing the release in a few days; you can go get the HEAD of that branch in the meantime.

I'm going to close this for now, as the bug is already fixed. Thanks!