Don't create a new array for empty/null match every time

If PREG_OFFSET_CAPTURE is used, unmatched subpatterns will be either
[null, -1] or ['', -1] depending on PREG_UNMATCHED_AS_NULL mode.
Instead of creating a new array like this every time, cache it inside
a global (per-request -- could make it immutable though).

Additionally check whether the subpattern is an empty string or
single character string and use an existing interned string in that
case. Empty / single-char subpatterns are common, so let's avoid
allocating strings for them.

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