fix(migrator): add nil guards to ColumnType methods to prevent panic (#7767)

When a driver populates ColumnType from information_schema but fails to
attach the *sql.ColumnType, calling DecimalSize(), Length(), Nullable(),
ScanType(), Name(), or DatabaseTypeName() panics on nil dereference.

Add nil checks before delegating to SQLColumnType in all 6 methods,
returning zero values when SQLColumnType is nil.

Fixes #7764