refactor(el): Use the fact that a timer callback must be set in ua_timer.c
refactor(el): Use the fact that a timer callback must be set in ua_timer.c

open62541/open62541,JGrothoff/open62541,open62541/open62541,JGrothoff/open62541,JGrothoff/open62541,open62541/open62541,open62541/open62541,JGrothoff/open62541