Getting a `source` with a nullable value confuses type checker

[achia70]

Vide 0.4.0 from Wally

Trying to get a source with a nullable value results in a type error, like this:

local someText = Vide.source(nil :: string?)

print(someText()) -- TypeError: Calling function (() -> string?) & ((string?) -> string?) with argument pack () is ambiguous.

someText("afjwirinefirwiuwefui")

if someText() then -- TypeError: Calling function (() -> string?) & ((string?) -> string?) with argument pack () is ambiguous.
	print(someText()) -- TypeError: Calling function (() -> string?) & ((string?) -> string?) with argument pack () is ambiguous.
end

This requires explicit casting to silent out the error:

print((someText :: () -> string?)())

someText("afjwirinefirwiuwefui")

if (someText :: () -> string?)() then
	print((someText :: () -> string)())
end

[centau]

I've made sources like that in my own code before and don't remember getting that error. When I check again now I see the error too. I opened an issue on the luau repo: luau-lang/luau#2362