Fixing nullability issue of message and adding test coverage

.changes/next-release/bugfix-AWSSDKforJavav2-4234091.json

@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "AWS SDK for Java v2",
+    "contributor": "",
+    "description": "Fixed an issue in AwsServiceException#getMessage() where it returned an empty string instead of null when the message is null."
+}

core/aws-core/src/main/java/software/amazon/awssdk/awscore/exception/AwsServiceException.java

@@ -79,7 +79,8 @@ public String getMessage() {
             joiner.add(diagnostics.toString());
         }
 
-        return joiner.toString();
+        String result = joiner.toString();
+        return result.isEmpty() ? super.getMessage() : result;
     }
 
     private String serviceDiagnostics() {

core/aws-core/src/test/java/software/amazon/awssdk/awscore/exception/AwsServiceExceptionTest.java

@@ -122,6 +122,32 @@ private static Stream<Arguments> exceptionMessageTestCases() {
         );
     }
 
+    @ParameterizedTest
+    @MethodSource("messageNullabilityTestCases")
+    void getMessage_respectsNullBehavior(String testDescription,
+                                         AwsServiceException.Builder builder,
+                                         String expectedMessage) {
+        AwsServiceException e = builder.build();
+        assertThat(e.getMessage())
+            .as(testDescription)
+            .isEqualTo(expectedMessage);
+    }
+
+    private static Stream<Arguments> messageNullabilityTestCases() {
+        return Stream.of(
+            Arguments.of(
+                "No message or details set",
+                AwsServiceException.builder(),
+                null
+            ),
+            Arguments.of(
+                "Explicitly null message without details",
+                AwsServiceException.builder().message(null),
+                null
+            )
+        );
+    }
+
     @Test
     public void exceptionMessage_withoutErrorMessage() {
         AwsServiceException e = AwsServiceException.builder()