Annotate methods that return a definitely null value with @Nullable

We intend to enable the ReturnMissingNullable errorprone pattern in
Gerrit core and this fixes the current issues.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I35cdd340aff94b8111b3864a676ecd9a9b4409af

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 2e08c11..da654a5 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java
@@ -103,6 +103,7 @@
     return new ListGroupMembership(groups.build());
   }
 
+  @Nullable
   @Override
   public GroupDescription.Basic get(AccountGroup.UUID uuid) {
     String ident = username(uuid);