tests: no need to merge operators when NotSupportError raised

[waketzheng]

Action https://github.com/tortoise/aerich/actions/runs/12142921728/job/33858554690 failed with the following info:

>           assert Migrate.upgrade_operators == []
E           assert ['ALTER TABLE...itle_f7fc03"'] == []
E             
E             Left contains one more item: 'ALTER TABLE "category" DROP INDEX "uid_category_title_f7fc03"'
E             
E             Full diff:
E             - []
E             + [
E             +     'ALTER TABLE "category" DROP INDEX "uid_category_title_f7fc03"',
E             + ]

tests/test_migrate.py:1038: AssertionError
=========================== short test summary info ============================
FAILED tests/test_migrate.py::test_migrate - assert ['ALTER TABLE...itle_f7fc03"'] == []
  
  Left contains one more item: 'ALTER TABLE "category" DROP INDEX "uid_category_title_f7fc03"'
  
  Full diff:
  - []
  + [
  +     'ALTER TABLE "category" DROP INDEX "uid_category_title_f7fc03"',
  + ]
========================= 1 failed, 15 passed in 0.25s =========================
make: *** [Makefile:36: test_sqlite] Error 1

That is because the ALTER operator add to class var of Migrate before NotSupportError raised.

Resolve it by move the Migrate._merge_operators() to only run in case of ddl is not sqlite.