migrationCallbacks run every time, not only when outstanding migrations are detected

[robertoschwald]

in 1.x, migrationCallbacks.beforeStartMigration was triggered only when outstanding migrations were detected.
Now it runs every time.

This means e.g. automatic db backup using a callback bean is not possible anymore (or needs back-porting of the old behaviour into the callback bean).

See here how it behaved before:

https://github.com/grails-plugins/grails-database-migration/blob/33a93c7012caa15d2c209263ae3b943ea4e15d46/src/groovy/grails/plugin/databasemigration/MigrationRunner.groovy#L107

and now:
https://github.com/grails-plugins/grails-database-migration/blob/00c35ad00c2944c9128269c8ed2956f3443993c3/src/main/groovy/org/grails/plugins/databasemigration/liquibase/GrailsLiquibase.groovy#L90