MySQL: REGEXP/RLIKE followed by NULL is rejected or misparsed (regression in 0.59.0) #2760

Since 0.59.0, a NULL pattern after REGEXP or RLIKE is consumed as if it were NOT NULL, and the next token becomes the pattern:

SELECT 'abc' REGEXP NULL
  -> sql parser error: Expected: an expression, found: EOF

SELECT a RLIKE NULL FROM t
  -> displays as: SELECT a RLIKE FROM AS t      (pattern is the identifier FROM, no FROM clause)

SELECT a NOT REGEXP NULL OR b FROM t
  -> displays as: SELECT a NOT REGEXP OR AS b FROM t

In 0.58.0 these parsed as Expr::RLike { pattern: Value(Null), .. } and round-tripped.

Tested on main e15e9ab (also 0.63.0; 0.58.0 is fine).

Found with a property-based fuzzer (round trip and statement-separation checks); I used AI assistance for triage and verified every case above.