fix: fix crash extracting struct with type parameter

[matt2e]

FTL panics when compiling code like this:

//ftl:data
type DataWithType[T any] struct {
	Value T
}

VisitStruct was iterating through type parameters and type parameter values, but there are no values in this case.

[matt2e]

Actually, putting this on hold, I think #1522 might have unblocked what I needed.

[matt2e]

It wasn't blocking me in the end, but submitting again because I do think its a legit issue.