Construct non-NULL regexps from NULL ranges (for variable-length encodings).

Construct non-NULL regexps from NULL ranges (for variable-length encodings).

NULL range represents empty range: range union and difference functions
return NULL for empty ranges. Thus NULL can be passed to functions
that construct regexp from range ('MatchOp', 'UTF8Range' and 'UTF16Range').
All these functions must behave return non-NULL for NULL ranges, since
further code relies on this.