Register event handlers on draft service, only if at least one draft service is available

[mofterdinger]

Register event handlers on draft service, only if at least one draft service is available. This avoids warning on application startup:

WARN 44101 --- [  restartedMain] c.s.c.s.i.h.HandlerRegistryTools         : Failed to register handler method 'public void com.sap.cds.feature.attachments.handler.draftservice.DraftPatchAttachmentsHandler.processBeforeDraftPatch(com.sap.cds.services.draft.DraftPatchEventContext,java.util.List)': Could not find any matching service.
WARN 44101 --- [  restartedMain] c.s.c.s.i.h.HandlerRegistryTools         : Failed to register handler method 'public void com.sap.cds.feature.attachments.handler.draftservice.DraftCancelAttachmentsHandler.processBeforeDraftCancel(com.sap.cds.services.draft.DraftCancelEventContext)': Could not find any matching service.
WARN 44101 --- [  restartedMain] c.s.c.s.i.h.HandlerRegistryTools         : Failed to register handler method 'public void com.sap.cds.feature.attachments.handler.draftservice.DraftActiveAttachmentsHandler.processDraftSave(com.sap.cds.services.draft.DraftSaveEventContext)': Could not find any matching service.