`@Contract` for `SupplierUtils.resolve(Supplier)` incorrectly declares `!null -> !null`

[sbrannen]

The nullability @Contract declaration for SupplierUtils.resolve(Supplier) currently declares !null -> !null.

However, invoking Supplier#get() can return null.

Thus, that part of the contract declaration is incorrect, which can lead to failures for nullability checks.

We should therefore remove that part of the contract.