Subscribing on minimal ExtRef definition

[JakobVogelsang]

As a user of OpenSCD I want to be able to see already existing subscriptions in OpenSCD in all circumstances. At the moment when an ExtRef element has missing srcLNClass and srcLNInst is interpreted as unsubscribe. That however is incorrect as described here.

Acceptance Criteria On checking existing subscriptions, the existing rules must be extended so that:

• have srcLNClass defaulting to LLN0 when missing
• have srcLNInst defaulting to empty string when missing

Example <ExtRef iedName="AA1E1Q01BCU" ldInst="MV" ... srcLDInst="LD0" srcCBName="gcb" serviceType="GOOSE" /> must be interpreted as <ExtRef iedName="AA1E1Q01BCU" ldInst="MV" ... srcLDInst="LD0" srcLNClass="LLN0" srcLNInst="" srcCBName="gcb" serviceType="GOOSE" />