Use List.copyOf() instead of Collections.unmodifiableList()

[bazzi2548]

We can safely use List.copyOf() because we check if (providers) is null.

If use List.copyOf()

• Clean code
• Performance optimization
• Guarantees a truly immutable list.

[philwebb]

I think this is probably safe, but we need to be a little careful using List.copyOf. Unlike Collections.unmodifiableList(new ArrayList<>(...)) it will throw an exception if any of the elements are null.

[philwebb]

Thanks @bazzi2548!