fix nullaway warnings

Author: msridhar

nullaway/src/main/java/com/uber/nullaway/generics/GenericsChecks.java

@@ -1706,6 +1706,9 @@ public void checkTypeParameterNullnessForFunctionReturnType(
                 formalReturnType,
                 false);
       }
+      if (returnExpressionType == null) {
+        return;
+      }
       boolean isReturnTypeValid =
           subtypeParameterNullability(formalReturnType, returnExpressionType, state);
       if (!isReturnTypeValid) {
@@ -2105,6 +2108,9 @@ public void compareGenericTypeParameterNullabilityForCall(
                           formalParameter,
                           false);
                 }
+                if (actualParameterType == null) {
+                  return;
+                }
                 if (!subtypeParameterNullability(formalParameter, actualParameterType, state)) {
                   reportInvalidParametersNullabilityError(
                       formalParameter, actualParameterType, currentActualParam, state);