REGEXP_LIKE does not error out invalid pattern strings

In N1QL's current implementation of REGEXP_LIKE, "^" and "$" is added for the exact match:
https://github.com/couchbase/query/blob/master/expression/func_regexp.go#L168

Therefore, certain invalid regular expression pattern strings that should be rejected can be accepted.

Example:
SELECT REGEXP_LIKE('Steven', '*');
Result:
[

]

MySQL raises an error for that.
mysql> SELECT 'Steven' rlike '*';
ERROR 1139 (42000): Got error 'repetition-operator operand invalid' from regexp