fix(event-scripts): wire scriptMethod selection + fallback on empty name

Two bugs prevented saving a new event script:

1. The Script Method <mat-select> had no (selectionChange) handler, so
   selectedRoute() never fired. completeScriptName was therefore never
   populated from the chosen method; it remained the empty string that
   selectedServiceItemEvent() resets it to.

2. The submit fallback used `??`, which only treats null/undefined as
   missing. The empty-string completeScriptName passed through unchanged,
   so scriptItem.name became '' and the POST went to /system/event_script
   (no resource id) — backend then rejects with 400 "No record(s) detected".

Wire selectedRoute() to scriptMethod's selectionChange and use || so the
fallback also kicks in for empty string.

Author: nicdavidson