XTCE12 — StringAlarmList is required but it should be optional

This is similar to issue 19370. StringParameterTypes can have several types of alarms but the StringAlarmList is required. Thus if you have a string alarm condition but nothing to go into the string alarm list, you have to make one up. For example if you are comparing the current string value to a past one in some manner, there is not necessarily a string alarm to go with it. The solution is to make the string alarm list optional.

Allow for no StringAlarmList element in StringDataType

Propose to resolve to allow for StringAlarmList to be optional in the StringDataType definition. This does have the side effect of allowing DefaultAlarm to have empty content, which is perhaps not so bad.