fix: make local optional in typescript definitions

[jamsea]

@lazeratops and I noticed that local is sometimes undefined. Currently our workaround is to add a //@ts-ignore in the project we're working on. This should fix this issue.

[lazeratops]

The change itself LGTM, but I think we should get some added feedback from product eng as well. I am wondering if we need to take any special considerations of backwards compatibility here, since I suppose this would could break transpiration for upgrading typescript customers who may not already check for truthiness.

(Also I think we may want to use the main branch as the merge target here, unless the workflow has changed)

[mattieruth]

i'll let @kompfner weigh in here, but I think maybe the more accurate thing to do would be to change the return of participants()(line 1105) to return DailyParticipantsObject || {}. The only time local can be undefined is before preAuth/startCamera/join or after leave (basically.. not in a call) and in this case, participants() will return {}. The other uses of DailyParticipantsObject come at times when you are in a call and should be guaranteed to have a local participant. With the change I'm proposing, we can keep this guarantee.