AccountCacheImpl: Don't return null on error but Optional.empty()

Callers do not expect that methods that return an Optional may also
return null.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I1962891693aaa9048771f5f7eae7c303a826da13

java/com/google/gerrit/server/account/AccountCacheImpl.java

@@ -101,7 +101,7 @@ public class AccountCacheImpl implements AccountCache {
       return byId.get(accountId);
     } catch (ExecutionException e) {
       logger.atWarning().withCause(e).log("Cannot load AccountState for ID %s", accountId);
-      return null;
+      return Optional.empty();
     }
   }
 
@@ -149,7 +149,7 @@ public class AccountCacheImpl implements AccountCache {
           .orElseGet(Optional::empty);
     } catch (IOException | ConfigInvalidException e) {
       logger.atWarning().withCause(e).log("Cannot load AccountState for username %s", username);
-      return null;
+      return Optional.empty();
     }
   }