string: Allow collect --allow-empty to avoid empty ellision (#8054)

* string: Allow `collect --no-empty` to avoid empty ellision

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` to be used, in which case we
print one empty argument.

This means

    test -n (thing | string collect -n)

can now be safely used.

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

* Switch to `--allow-empty`

`--no-empty` does the exact opposite for `string split` and split0.

Since `-a`/`--allow-empty` already exists, use it.

tests/checks/string.fish

@@ -667,6 +667,18 @@ string collect -N '' >/dev/null; and echo unexpected success; or echo expected f
 string collect \n\n >/dev/null; and echo unexpected success; or echo expected failure
 # CHECK: expected failure
 
+echo "foo"(true | string collect --allow-empty)"bar"
+# CHECK: foobar
+test -z (string collect)
+and echo Nothing
+# CHECK: Nothing
+test -n (string collect)
+and echo Something
+# CHECK: Something
+test -n (string collect -a)
+or echo No, actually nothing
+# CHECK: No, actually nothing
+
 # string collect in functions
 # This function outputs some newline-separated content, and some
 # explicitly un-separated content.