Adapt to changes in Gerrit API

IdentifiedUser#getByUserName now returns Optional<String> insteadof
a nullable String.

Change-Id: If0176d1a1f2c08a06702c19478a150d199e7133e
Signed-off-by: Edwin Kempin <ekempin@google.com>

src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java b/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java
index 7d495e8..be2c749 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java
@@ -95,8 +95,8 @@
   public GroupMembership membershipsOf(IdentifiedUser user) {
     ImmutableList.Builder<AccountGroup.UUID> groups = ImmutableList.builder();
     groups.add(uuid(user.getAccountId()));
-    if (user.getUserName() != null) {
-      groups.add(uuid(user.getUserName()));
+    if (user.getUserName().isPresent()) {
+      groups.add(uuid(user.getUserName().get()));
     }
     return new ListGroupMembership(groups.build());
   }