feat(core): The timer ignores entries with no callback
feat(core): The timer ignores entries with no callback

jpfr/open62541,JGrothoff/open62541,StalderT/open62541,jpfr/open62541,jpfr/open62541,open62541/open62541,JGrothoff/open62541,JGrothoff/open62541,jpfr/open62541,StalderT/open62541,StalderT/open62541,open62541/open62541,StalderT/open62541,JGrothoff/open62541,open62541/open62541,open62541/open62541