Properly handle removal od "main component name"

If an application version already has a "main component name" specified and it is removed, subsequent deployments of the app will fail.

This happens because removing an existing main component name sets the appropriate value in the database to an empty string instead of NULL, as expected and as set by default if the value is omitted during initial app definition.

This leads to the platform looking for deployment/statefulset names such as tests-netbox-215-.