Turning REGEXP_REPLACE into two schema-qualified functions:
- mariadb_schema.regexp_replace()
- oracle_schema.regexp_replace()

Fixing oracle_schema.regexp_replace(subj,pattern,replacement) to treat
NULL in "replacement" as an empty string.

Adding new classes implementing oracle_schema.regexp_replace():
- Item_func_regexp_replace_oracle
- Create_func_regexp_replace_oracle

Adding helper methods:
- String *Item::val_str_null_to_empty(String *to)
- String *Item::val_str_null_to_empty(String *to, bool null_to_empty)

and reusing these methods in both Item_func_replace and
Item_func_regexp_replace.