Class existence inference in BAML source type hints

[anerli]

With a class as an optional param to a BAML source function, if we check that the variable is populated, we still get a warning saying it could be none.

Example:

function Foo (bar: Baz?) -> string {
	client MyClient
	prompt #"
		{ %if bar }
			{{ bar.x }} // Gives 'bar' is a (none | class Baz), expected class
		{ %endif% }
	"#
}