Fix split/1 with empty string, just as string division

[itchyny]

This PR fixes split/1 to work with empty string. This filter is semantically same as string division so I refactored not to duplicate code.

❯ jq -nc '"abc" | split("b", "")'
["a","c"]
["a","b","c"]

❯ jq -nc '"abc" / ("b", "")'
["a","c"]
["a","b","c"]

❯ xq -nc '"abc" | split("b", "")'
["a","c"]
["abc"]  # oops!

❯ xq -nc '"abc" / ("b", "")'
["a","c"]
["a","b","c"]  # ok!

❯ xq --version
xq 0.2.9-f33b40cf049d2ef44a4fa0f75f92efc9255fda62

[MiSawa]

Looks good, thank you!