Fix Java 8 build and avoid Optional.isEmpty()

Gerrit v3.4 should still be source compatible with Java 8
therefore the Java 11's Optional.isEmpty() should be replaced
with !Optional.isPresent().

Change-Id: I52491602086c0df8b39e1ee95c5e53c21d10042d

src/main/java/com/googlesource/gerrit/plugins/multisite/validation/MultisiteReplicationFetchFilter.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/validation/MultisiteReplicationFetchFilter.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/validation/MultisiteReplicationFetchFilter.java
index d355984..21d4073 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/validation/MultisiteReplicationFetchFilter.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/validation/MultisiteReplicationFetchFilter.java
@@ -76,7 +76,7 @@
                             ref,
                             oid.getName()));
 
-                return localRefOid.isEmpty();
+                return !localRefOid.isPresent();
               })
           .collect(Collectors.toSet());
     } catch (IOException ioe) {
@@ -129,7 +129,7 @@
                           .orElse(false))
               .map(Ref::getObjectId);
 
-      if (localRefObjectId.isEmpty() && retryWithRandomSleep) {
+      if (!localRefObjectId.isPresent() && retryWithRandomSleep) {
         randomSleepForMitigatingConditionWhereLocalRefHaveJustBeenChanged(
             projectName, localRefObjectId, ref);
         localRefObjectId =