Do not complain about empty auto-index file

AppEngineRule throws an exception during teardown if
an empty datastore-indexes-auto.xml is created during
testing. This scenario should be fine, just like if this file
has not been created at all.

This issue causes some flakiness in tests.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=231229692

javatests/google/registry/testing/AppEngineRuleTest.java

@@ -0,0 +1,95 @@
+// Copyright 2019 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.testing;
+
+import static google.registry.testing.JUnitBackports.assertThrows;
+import static java.nio.charset.StandardCharsets.UTF_8;
+
+import com.google.common.base.Joiner;
+import java.io.File;
+import java.io.IOException;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/**
+ * Unit tests for {@link google.registry.testing.AppEngineRule}. Tests focus on Datastoe-related
+ * assertions made during teardown checks.
+ */
+@RunWith(JUnit4.class)
+public class AppEngineRuleTest {
+
+  // An arbitrary index in google/registry/env/common/default/WEB-INF/datastore-indexes.xml
+  private static final String DECLARED_INDEX =
+      Joiner.on('\n')
+          .join(
+              "<datastore-indexes autoGenerate=\"false\">",
+              "  <datastore-index kind=\"ContactResource\" ancestor=\"false\" source=\"manual\">",
+              "    <property name=\"currentSponsorClientId\" direction=\"asc\"/>",
+              "    <property name=\"deletionTime\" direction=\"asc\"/>",
+              "    <property name=\"searchName\" direction=\"asc\"/>",
+              "  </datastore-index>",
+              "</datastore-indexes>");
+  private static final String UNDECLARED_INDEX =
+      DECLARED_INDEX.replace("ContactResource", "NoSuchResource");
+
+  @Rule public TemporaryFolder temporaryFolder = new TemporaryFolder();
+
+  private final AppEngineRule appEngineRule = AppEngineRule.builder().withDatastore().build();
+
+  @Before
+  public void setupAppEngineRule() throws Exception {
+    appEngineRule.temporaryFolder = temporaryFolder;
+    appEngineRule.before();
+  }
+
+  @Test
+  public void testTeardown_successNoAutoIndexFile() {
+    appEngineRule.after();
+  }
+
+  @Test
+  public void testTeardown_successEmptyAutoIndexFile() throws Exception {
+    writeAutoIndexFile("");
+    appEngineRule.after();
+  }
+
+  @Test
+  public void testTeardown_successWhiteSpacesOnlyAutoIndexFile() throws Exception {
+    writeAutoIndexFile("  ");
+    appEngineRule.after();
+  }
+
+  @Test
+  public void testTeardown_successOnlyDeclaredIndexesUsed() throws Exception {
+    writeAutoIndexFile(DECLARED_INDEX);
+    appEngineRule.after();
+  }
+
+  @Test
+  public void testTeardown_failureUndeclaredIndexesUsed() throws Exception {
+    writeAutoIndexFile(UNDECLARED_INDEX);
+    assertThrows(AssertionError.class, () -> appEngineRule.after());
+  }
+
+  private void writeAutoIndexFile(String content) throws IOException {
+    com.google.common.io.Files.asCharSink(
+            new File(temporaryFolder.getRoot(), "datastore-indexes-auto.xml"), UTF_8)
+        .write(content);
+  }
+}