Add a case in our inference for generic method calls (uber#1240)

Fixes uber#1238 

Before we were missing a fallback case; now we just use the type of the
expression computed by `javac` (to be improved in the future).

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved handling of nullable type variables in generic methods to
ensure correct type inference in assignment contexts.

* **Tests**
* Added a new test to verify correct behavior when assigning
non-nullable arrays to nullable array parameters in generic methods.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: msridhar

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

@@ -514,6 +514,10 @@ private Type inferGenericMethodCallType(
             // TODO we should just strip out the top-level @Nullable annotation;
             //  stripMetadata() also removes nested @Nullable annotations
             substitution.put(typeVar, typeFromAssignmentContext.stripMetadata());
+          } else {
+            // Just stick with the original javac type of the expression.  As we improve inference
+            // we will do better.
+            substitution.put(typeVar, exprType);
           }
         }
 

nullaway/src/test/java/com/uber/nullaway/jspecify/GenericMethodTests.java

@@ -738,6 +738,43 @@ public void typeVarReturnNonNullUpperBound() {
         .doTest();
   }
 
+  @Test
+  public void issue1238() {
+    makeHelper()
+        .addSourceLines(
+            "CreatorMediator.java",
+            "import org.jspecify.annotations.NullMarked;",
+            "import org.jspecify.annotations.Nullable;",
+            "@NullMarked",
+            "public class CreatorMediator {",
+            "    private final PropertyModel mCreatorModel = new PropertyModel();",
+            "    private void followClickHandler() {",
+            "        // this is fine: byte[] is assignable to byte @Nullable []",
+            "        WebFeedBridge.followFromId(",
+            "                mCreatorModel.get(CreatorProperties.WEB_FEED_ID_KEY));",
+            "    }",
+            "    private static class PropertyModel {",
+            "        <T extends @Nullable Object> T get(Key<T> key) {",
+            "            throw new RuntimeException();",
+            "        }",
+            "    }",
+            "    private static class CreatorProperties {",
+            "        static final Key<byte[]> WEB_FEED_ID_KEY = makeKey();",
+            "        private static Key<byte[]> makeKey() {",
+            "            throw new RuntimeException();",
+            "        }",
+            "    }",
+            "    private static class WebFeedBridge {",
+            "        static void followFromId(",
+            "                byte @Nullable [] webFeedId) {",
+            "        }",
+            "    }",
+            "    interface Key<T> {",
+            "    }",
+            "}")
+        .doTest();
+  }
+
   private CompilationTestHelper makeHelper() {
     return makeTestHelperWithArgs(
         Arrays.asList(