String helper `split` doesn't split by empty string.

[mbehzad]

Hi,
split helper will ignore the empty string ("") as the separator and fall back to comma (,) which is a different behavior compared to the JS's String.prototype.split method. e.g.:
{{split "text" ""}} -> ["text"] but "text".split("") -> ["t", "e", "x", "t"]

The issue with util.isString and it returning false for the empty string was previously discussed in issue #273 . but i think for this helper it makes sense to have different logic.

I can provide a PR if you are open to it.