[CX] Make string functions return NULL on invalid arguments

string-equal

matches

regexp-like

regexp-position

regexp-position1

regexp-replace

regexp-matches

regexp-split

lowercase

uppercase

initcap

trim

ltrim

rtrim

position

position1

replace

reverse

string-length

like

contains

starts-with

ends-with

substring

substring1

substring-before

substring-after

string-to-codepoint

codepoint-to-string

string-concatconcat2

string-join

repeat

split