Modify ARN toString to print a valid ARN when there's no region or acountId

.changes/next-release/bugfix-AWSSDKforJavav2-51797eb.json

@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "AWS SDK for Java v2",
+    "contributor": "Madrigal",
+    "description": "Modify ARN toString to print a valid ARN when there's no region or acountId"
+}

core/arns/src/main/java/software/amazon/awssdk/arns/Arn.java

@@ -198,9 +198,9 @@ public String toString() {
                + ":"
                + this.service
                + ":"
-               + region
+               + (region == null ? "" : region)
                + ":"
-               + this.accountId
+               + (this.accountId == null ? "" : this.accountId)
                + ":"
                + this.resource;
     }

core/arns/src/test/java/software/amazon/awssdk/arns/ArnTest.java

@@ -57,6 +57,22 @@ public void arnWithQualifiedResource_ParsesBackToString() {
         assertThat(arn.resourceAsString()).isEqualTo("myresource:foobar:1");
     }
 
+    @Test
+    public void arnWithMinimalResources_ParsesBackToString() {
+        String arnString = "arn:aws:s3:::bucket";
+        Arn arn = Arn.fromString(arnString);
+        assertThat(arn.toString()).isEqualTo(arnString);
+        assertThat(arn.resourceAsString()).isEqualTo("bucket");
+    }
+
+    @Test
+    public void arnWithoutRegion_ParsesBackToString() {
+        String arnString = "arn:aws:iam::123456789012:root";
+        Arn arn = Arn.fromString(arnString);
+        assertThat(arn.toString()).isEqualTo(arnString);
+        assertThat(arn.resourceAsString()).isEqualTo("root");
+    }
+
     @Test
     public void arnWithResourceTypeAndResource_ParsesCorrectly() {
         String arnString = "arn:aws:s3:us-east-1:12345678910:bucket:foobar";