Delete any leftover usernames in the accounts db

The account username field should not currently be populated

service/src/test/java/org/whispersystems/textsecuregcm/storage/UsernameCleanerTest.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2022 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+package org.whispersystems.textsecuregcm.storage;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.*;
+import static org.mockito.Mockito.*;
+
+class UsernameCleanerTest {
+
+  private final AccountsManager accountsManager = mock(AccountsManager.class);
+  private final Account hasUsername = mock(Account.class);
+  private final Account noUsername = mock(Account.class);
+
+
+  @BeforeEach
+  void setup() {
+    when(hasUsername.getUsername()).thenReturn(Optional.of("n00bkiller"));
+    when(noUsername.getUsername()).thenReturn(Optional.empty());
+  }
+
+  @Test
+  void testAccounts() throws AccountDatabaseCrawlerRestartException {
+    UsernameCleaner accountCleaner = new UsernameCleaner(accountsManager);
+    accountCleaner.onCrawlStart();
+    accountCleaner.timeAndProcessCrawlChunk(Optional.empty(), Arrays.asList(hasUsername, noUsername));
+    accountCleaner.onCrawlEnd(Optional.empty());
+    verify(accountsManager).clearUsername(hasUsername);
+    verify(accountsManager, never()).clearUsername(noUsername);
+  }
+}