Missing check for library models for lambdas with an expression body

In our handling of lambdas, we immediately bail out of all checking if the corresponding functional interface method is unannotated:

https://github.com/uber/NullAway/blob/084bd96ff473252715229fc41e02ac11f6f2bfd1/nullaway/src/main/java/com/uber/nullaway/NullAway.java#L957-L959

But, this skips a check that is later performed for return types, where we allow library models to override return type nullability:

https://github.com/uber/NullAway/blob/084bd96ff473252715229fc41e02ac11f6f2bfd1/nullaway/src/main/java/com/uber/nullaway/NullAway.java#L875-L877

In particular, we model Guava Functions as having a @NonNull return, but if you write (x) -> null and pass it as a Guava Function currently NullAway does not report an error. In contrast, writing (x) -> { return null; } yields an error. This should be relatively easy to fix, but it will lead to new errors when users do a NullAway update.

Start in nullaway/src/main/java/com/uber/nullaway/NullAway.java around lines 957-959 and compare the lambda handling with lines 875-877, where library models can override return nullability. Reproduce the Guava Function cases using expression and block lambda bodies. Done means an expression-bodied lambda returning null is checked consistently with the block-bodied form, with regression coverage added where appropriate.