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: Id502fba86964c0c9b8df5fd586a1d98aa53137bf

src/main/java/com/googlesource/gerrit/plugins/reviewnotes/CreateReviewNotes.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/CreateReviewNotes.java b/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/CreateReviewNotes.java
index 03147fa..7b850ff 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/CreateReviewNotes.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/CreateReviewNotes.java
@@ -17,6 +17,7 @@
 import static java.nio.charset.StandardCharsets.UTF_8;
 
 import com.google.common.flogger.FluentLogger;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.entities.Change;
 import com.google.gerrit.entities.LabelType;
 import com.google.gerrit.entities.LabelTypes;
@@ -224,6 +225,7 @@
     }
   }
 
+  @Nullable
   private ObjectId createNoteContent(ChangeNotes notes, PatchSet ps) throws IOException {
     HeaderFormatter fmt = new HeaderFormatter(gerritServerIdent.getTimeZone(), anonymousCowardName);
     if (ps != null) {
@@ -237,6 +239,7 @@
     return null;
   }
 
+  @Nullable
   private PatchSet loadPatchSet(RevCommit c, String destBranch) {
     String hash = c.name();
     for (ChangeData cd : queryProvider.get().byBranchCommit(project.get(), destBranch, hash)) {

src/main/java/com/googlesource/gerrit/plugins/reviewnotes/ExportReviewNotes.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/ExportReviewNotes.java b/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/ExportReviewNotes.java
index e8ece83..1a097c1 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/ExportReviewNotes.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/ExportReviewNotes.java
@@ -20,6 +20,7 @@
 import com.google.common.collect.ListMultimap;
 import com.google.common.collect.Maps;
 import com.google.common.collect.MultimapBuilder;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.server.git.GitRepositoryManager;
@@ -113,6 +114,7 @@
         .call();
   }
 
+  @Nullable
   private Map.Entry<Project.NameKey, List<ChangeNotes>> next() {
     synchronized (lock) {
       if (changes.isEmpty()) {