Replace uses of `Locations[0]` and `Locations.First()`

[jcouv]

We should replace them with an extension method that handles Locations being an empty array: FirstOrNone().

Relates to #29180

[jaredpar]

Can you clarify this bug a bit? Is this specific to NRT or is it a more general requirement for the compiler?

[jcouv]

NRT hit this when we had NonNullTypesAttribute (since the injected type didn't have syntax and therefore no location).
I wouldn't consider urgent/blocking anymore. But it's good hygiene to make the change at some point.
Removed the NRT label

[jaredpar]

I agree this is good hygiene to be doing here and we should keep this open as a general issue.