keep default settings

Author: mpeddada1

oauth2_http/java/com/google/auth/oauth2/ServiceAccountJwtAccessCredentials.java

@@ -112,7 +112,7 @@ private ServiceAccountJwtAccessCredentials(
         privateKeyId,
         null,
         null,
-        null);
+        Credentials.GOOGLE_DEFAULT_UNIVERSE);
   }
 
   /**
@@ -123,7 +123,8 @@ private ServiceAccountJwtAccessCredentials(
    * @param privateKey RSA private key object for the service account.
    * @param privateKeyId Private key identifier for the service account. May be null.
    * @param defaultAudience Audience to use if not provided by transport. May be null.
-   * @param universeDomain universe domain in the format some-domain.xyz.
+   * @param universeDomain universe domain in the format some-domain.xyz. By default, sets it to
+   *     googleapis.com
    */
   private ServiceAccountJwtAccessCredentials(
       String clientId,
@@ -140,7 +141,11 @@ private ServiceAccountJwtAccessCredentials(
     this.defaultAudience = defaultAudience;
     this.credentialsCache = createCache();
     this.quotaProjectId = quotaProjectId;
-    this.universeDomain = universeDomain;
+    if (universeDomain == null || universeDomain.trim().isEmpty()) {
+      this.universeDomain = Credentials.GOOGLE_DEFAULT_UNIVERSE;
+    } else {
+      this.universeDomain = universeDomain;
+    }
   }
 
   /**
@@ -234,7 +239,7 @@ public static ServiceAccountJwtAccessCredentials fromPkcs8(
         privateKeyId,
         defaultAudience,
         null,
-            Credentials.GOOGLE_DEFAULT_UNIVERSE);
+        Credentials.GOOGLE_DEFAULT_UNIVERSE);
   }
 
   static ServiceAccountJwtAccessCredentials fromPkcs8(

oauth2_http/javatests/com/google/auth/oauth2/ServiceAccountJwtAccessCredentialsTest.java

@@ -113,7 +113,7 @@ public void constructor_allParameters_constructs() throws IOException {
     assertEquals(privateKey, credentials.getPrivateKey());
     assertEquals(SA_PRIVATE_KEY_ID, credentials.getPrivateKeyId());
     assertEquals(QUOTA_PROJECT, credentials.getQuotaProjectId());
-    assertNull(credentials.getUniverseDomain());
+    assertEquals(Credentials.GOOGLE_DEFAULT_UNIVERSE, credentials.getUniverseDomain());
   }
 
   @Test
@@ -917,7 +917,7 @@ public void builder_defaultUniverseDomain() throws IOException {
             .setPrivateKeyId(SA_PRIVATE_KEY_ID)
             .setDefaultAudience(URI.create("default-audience"))
             .build();
-    assertNull(credentials.getUniverseDomain());
+    assertEquals(Credentials.GOOGLE_DEFAULT_UNIVERSE, credentials.getUniverseDomain());
   }
 
   @Test