fix(migrator): normalize DB-side "NULL" default to fix infinite AlterColumn loop on Postgres

[boobusy]

• Do only one thing
• Non breaking API changes
• Tested

Description

Normalize DB-side DEFAULT NULL literal to prevent infinite AlterColumn loop on PostgreSQL.

What did this pull request do?

• Add !strings.EqualFold(dv, "NULL") to the first default value comparison case in MigrateColumn
• Mirrors the existing model-side normalization where default:NULL tag is treated as no-default
• Prevents AlterColumn from being triggered on every startup when a PostgreSQL column has DEFAULT NULL in its column_default catalog entry

User Case Description

PostgreSQL's information_schema.columns.column_default stores the literal string "NULL" for columns explicitly defined with DEFAULT NULL, while MySQL and other drivers return SQL NULL (empty) for the same semantic state.
GORM's MigrateColumn already normalizes the model side — default:NULL tag is treated as "no default" via EqualFold. But the DB side comparison was missing the same normalization, causing:

• dvNotNull = true (DB returns string "NULL")
• currentDefaultNotNull = false (model side normalized to no-default)
  This triggers AlterColumn on every startup. The postgres driver then issues ALTER COLUMN TYPE ... USING ... (full table rewrite holding AccessExclusiveLock) due to a separate alias-comparison bug, turning a metadata mismatch into a production outage on large tables.
  Affected drivers: PostgreSQL only. MySQL/SQLite/SQLServer all collapse DEFAULT NULL to "no default" in their column metadata.
  Related: AutoMigration always attempts to set the default value, even when it hasn’t changed. #7553, If a field in a database table uses the serializer tag and AutoMigrate is used, then even if there are no changes to this field, AutoMigrate will still perform an ALTER on the table every time #7590, fix(migrator): allow AutoMigrate to update explicit PK defaults #7769