Fix zero timestamp in vifl

[yushuqin]

Description

• MySQL returns "0000-00-00 00:00:00" for NULL timestamps when NO_ZERO_DATE sql_mode is not enabled
• The valueToVTTime() function in go/vt/vtctl/grpcvtctldserver/query.go attempts to parse this value using Go's standard time.ParseInLocation()
• Go's time parser rejects "0000-00-00 00:00:00" because "month 0" is invalid

Environment Difference(why only vifl keyspace is running into this issue):

• vifl shards: sql_mode = NO_ENGINE_SUBSTITUTION only → stores '0000-00-00 00:00:00'

dbadmin@tablet-iad-dev-vifl-0000-0020-gl0n[_vt]>   SELECT
  ->       migration_uuid,
  ->       migration_status,
  ->       last_throttled_timestamp,
  ->       last_throttled_timestamp = '0000-00-00 00:00:00' as is_zero_timestamp
  ->   FROM _vt.schema_migrations
  ->   WHERE migration_status = 'complete'
  ->   LIMIT 5;
+--------------------------------------+------------------+--------------------------+-------------------+
| migration_uuid                       | migration_status | last_throttled_timestamp | is_zero_timestamp |
+--------------------------------------+------------------+--------------------------+-------------------+
| afa730b3_ed9e_11f0_af0e_0affe767f54d | complete         | 0000-00-00 00:00:00      |                 1 |
| f7232069_ed9e_11f0_af0e_0affe767f54d | complete         | 0000-00-00 00:00:00      |                 1 |
| a43c6f0e_eda0_11f0_af0e_0affe767f54d | complete         | 0000-00-00 00:00:00      |                 1 |
| 4bd8c6e4_eda3_11f0_af0e_0affe767f54d | complete         | 0000-00-00 00:00:00      |                 1 |
| fe90f2a9_eda3_11f0_af0e_0affe767f54d | complete         | 0000-00-00 00:00:00      |                 1 |
+--------------------------------------+------------------+--------------------------+-------------------+
5 rows in set (0.00 sec)

• other shard: sql_mode includes STRICT_TRANS_TABLES → stores NULL

dbadmin@tablet-iad-staging-pool1-00-80-kx71[(none)]>   SELECT
    ->       migration_uuid,
    ->       migration_status,
    ->       last_throttled_timestamp,
    ->       last_throttled_timestamp = '0000-00-00 00:00:00' as is_zero_timestamp
    ->   FROM _vt.schema_migrations
    ->   WHERE migration_status = 'complete'
    ->   LIMIT 5;
+--------------------------------------+------------------+--------------------------+-------------------+
| migration_uuid                       | migration_status | last_throttled_timestamp | is_zero_timestamp |
+--------------------------------------+------------------+--------------------------+-------------------+
| 9ec5f164_cc41_11ee_a445_1264402010d7 | complete         | NULL                     |              NULL |
| abdb115b_cc41_11ee_a445_1264402010d7 | complete         | NULL                     |              NULL |
| dd68c4d5_e601_11ee_ad84_0e148289ab29 | complete         | NULL                     |              NULL |
| eea4bf57_e601_11ee_ad84_0e148289ab29 | complete         | NULL                     |              NULL |
| fd5aee11_e601_11ee_ad84_0e148289ab29 | complete         | NULL                     |              NULL |
+--------------------------------------+------------------+--------------------------+-------------------+
5 rows in set (0.01 sec)

  Error Call Stack

  vtops-go schema-change-executor monitor-change step
      ↓
      [gRPC call]
      ↓
  VtctldServer.GetSchemaMigrations()
      (go/vt/vtctl/grpcvtctldserver/server.go:1664)
      ↓
      [Query database: SELECT * FROM _vt.schema_migrations ...]
      ↓
  rowToSchemaMigration(row)
      (go/vt/vtctl/grpcvtctldserver/server.go:1771)
      ↓
  valueToVTTime(row.AsString("last_throttled_timestamp", ""))
      (go/vt/vtctl/grpcvtctldserver/query.go:154)
      ↓
  time.ParseInLocation("2006-01-02 15:04:05", "0000-00-00 00:00:00", time.Local)
      ↓
  ❌ ERROR: parsing time "0000-00-00 00:00:00": month out of range
      ↓
      [Error propagates back up the stack]
      ↓
  gRPC returns error with code = Unknown
      ↓
  vtops-go receives: "code = Unknown desc = parsing time ..."

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

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