ReplicationTasksStorage: Handle empty task files

Instead of NPEing if we manage to read a task file that's empty, return
an empty Optional.

Running the IT tests 1000 times produced this as the only failure (and
only once).

Change-Id: I3e7392dfb179795348d7f4a207102aa867aed85b

src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationTasksStorage.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationTasksStorage.java b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationTasksStorage.java
index 38a63ab..3947ebc 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationTasksStorage.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationTasksStorage.java
@@ -64,7 +64,7 @@
   public static class ReplicateRefUpdate {
     public static Optional<ReplicateRefUpdate> createOptionally(Path file) {
       try {
-        return Optional.of(create(file));
+        return Optional.ofNullable(create(file));
       } catch (NoSuchFileException e) {
         logger.atFine().log("File %s not found while reading task", file);
       } catch (IOException e) {