Fix internal errors when 'destination:' refers to non-existing destination

When querying with the destination: predicate using a destination name
that does not exist, it is supposed to return an error:

  "Unknown named destination"

However it was not, because of two problems:

- When the revision is null, VersionedAccountDestinations's call to
  getPathInfos results in NPE due to dereferencing the revision.

- When no destinations exist, DestinationList.getDestinations returns
  an empty list, but the ChangeQueryBuilder is only testing for null

Fix both of the bugs and add a test that querying "destination:" with
a non-existing destination results in the correct response.

Adding tests for existing destinations is not within the scope of this
change.

Change-Id: I7787a90a508f61ef19586f327c6be427a0870e69

gerrit-server/src/test/java/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java

@@ -2105,6 +2105,16 @@ public abstract class AbstractQueryChangesTest extends GerritServerTests {
     assertQuery("-assignee:" + user.getUserName(), change2);
   }
 
+  @Test
+  public void userDestination() throws Exception {
+    TestRepository<Repo> repo = createProject("repo");
+    insert(repo, newChange(repo));
+
+    assertThatQueryException("destination:foo")
+        .hasMessageThat()
+        .isEqualTo("Unknown named destination: foo");
+  }
+
   protected ChangeInserter newChange(TestRepository<Repo> repo) throws Exception {
     return newChange(repo, null, null, null, null);
   }