string: Allow collect --allow-empty to avoid empty ellision

[faho]

Currently we still have that issue where

test -n (thing | string collect)

can return true if thing doesn't print anything, because the
collected argument will still be removed.

So, what we do is allow --no-empty --allow-empty to be used, in which case we
print one empty argument.

This means

test -n (thing | string collect -a)

can now be safely used.

"noallow-empty" isn't the best name for this flag, but string's design
really incentivizes reusing names, and it's not terrible.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[faho]

"no-empty" isn't the best name for this flag, but string's design
really incentivizes reusing names, and it's not terrible.

I have now switched to using "--allow-empty" - because "--no-empty" does the opposite in split/split0 (it causes them to remove empty arguments)

[ridiculousfish]

An alternative to this is to support quoted substitutions, old #159:

test -n "$(thing | string collect)"

the advantage here is it works for all commands, not just string. But I don't mind --no-empty

[faho]

An alternative to this is to support quoted substitutions, old #159:

Oh, sure. And that's something I'd eventually like to have. But I've looked into it, and it's not trivial to add.

While this exists now.

[ridiculousfish]

Ok, seems reasonable; go for it.