Replace and replace_regex should allow empty strings as parameters

Basically, the following method call would work: $replace("abc" "a" "0"), while this would fail:

echo $replace("abc" "a" "")
ion: expansion error: replace: two arguments are required

The same goes for replace_regex. I think overall the methods should allow for empty strings as parameters, but check them later and fail, if it is not an appropriate argument.

Compare this to python:

python -c 'print("abc".replace("a", ""))'

and to sed:

echo abc | sed 's/a//g'

The same issue was raised here.

Overall, I can try to fix that later if the change makes sense.