allow app id to be none since it may be called during create

[barnjamin]

While answering a question in the forum, I realized I'd not considered the case where a MethodCall may be used to create an application.

Since the AVM doesn't like the manual setting of AppId 0, we should allow app_id to be None and prevent setting the app id field

[jasonpaulos]

Good catch. The solution looks good but it'd be nice to see a regression test added to test_InnerTxnBuilder_method_call

[tzaffi]

Thanks for the fix. I'm expecting mypy to flag the fact that app_id could now be None. So, I recommend changing its signature to:

app_id: Optional[Expr]

[ahangsu]

looks good, I will approve after a test on app-id == None gets added

[ahangsu]

also add a line in CHANGELOG Fixed section about this fix

[ahangsu]

looks right to me