assign default endpoint (Azure#34278)

Author: mssfang

sdk/appconfiguration/azure-data-appconfiguration/src/main/java/com/azure/data/appconfiguration/ConfigurationClientBuilder.java

@@ -138,7 +138,6 @@ public final class ConfigurationClientBuilder implements
 
     private ClientOptions clientOptions;
     private String connectionString;
-    private ConfigurationClientCredentials credential;
     private TokenCredential tokenCredential;
 
     private String endpoint;
@@ -210,7 +209,7 @@ public ConfigurationAsyncClient buildAsyncClient() {
      * authentication per builder instance.
      */
     private AzureAppConfigurationImpl buildInnerClient(SyncTokenPolicy syncTokenPolicy) {
-        String endpointLocal = null;
+        String endpointLocal = endpoint;
         ConfigurationClientCredentials credentialsLocal = null;
         TokenCredential tokenCredentialLocal = null;
         // validate the authentication setup

sdk/appconfiguration/azure-data-appconfiguration/src/test/java/com/azure/data/appconfiguration/ConfigurationClientBuilderTest.java

@@ -59,6 +59,17 @@ public void missingEndpoint() {
         });
     }
 
+    @Test
+    @DoNotRecord
+    public void clientMissingEndpointButTokenCredentialProvided() {
+        assertThrows(NullPointerException.class, () -> {
+            final ConfigurationClientBuilder builder = new ConfigurationClientBuilder();
+            TokenCredential credentials = request -> Mono.just(
+                new AccessToken("this_is_a_token", OffsetDateTime.MAX));
+            builder.credential(credentials).buildClient();
+        });
+    }
+
     @Test
     @DoNotRecord
     public void malformedURLExceptionForEndpoint() {