Missing check for library models for lambdas with an expression body

[msridhar]

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

NullAway/nullaway/src/main/java/com/uber/nullaway/NullAway.java

Lines 957 to 959 in 084bd96

	if (codeAnnotationInfo.isSymbolUnannotated(funcInterfaceMethod, config, handler)) {
	return Description.NO_MATCH;
	}

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

NullAway/nullaway/src/main/java/com/uber/nullaway/NullAway.java

Lines 875 to 877 in 084bd96

	return handler.onOverrideMethodReturnNullability(
	methodSymbol, state, isMethodAnnotated, methodReturnNullness);
	}

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.