Fixed bug#9456 - Empty 'Dropdown' dynamic field value of GenericAgent job wont get used and stored.
- Added missing perldoc for parameter 'Template' for EditFieldRender of Backend.pm.
- Improved empty scalar tolerance for dynamic field backends with 'PossibleNone' configuration for GenericAgent jobs.
- Fixed same bug for Multiselect backend.
- Fixed bug in 'BuildSelection': Emptry strings should be a valid value as 'SelectedID' parameter, if 'PossibleNone' is set, too.