t3200: replace hardcoded null OID with $ZERO_OID

t3200: replace hardcoded null OID with $ZERO_OID

To support the SHA-256 transition, replace the hardcoded 40-zero string
in 'git branch --merged' with '$ZERO_OID'. The current 40-character
string causes the test to fail prematurely in SHA-256 environments
because Git identifies a "malformed object name" (due to the 40 vs 64
character mismatch) before it even validates the object type.

By using '$ZERO_OID', we ensure the hash length is always correct for
the active algorithm. Additionally, use 'test_grep' to verify the
"must point to a commit" error message, ensuring the test validates
the object type logic rather than just string syntax.

Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Siddharth Shrimali <r.siddharth.shrimali@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>