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: I53a642afe69214799324f572c8d8737abc88c986

src/main/java/com/googlesource/gerrit/plugins/webhooks/DefaultHttpClientProvider.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/webhooks/DefaultHttpClientProvider.java b/src/main/java/com/googlesource/gerrit/plugins/webhooks/DefaultHttpClientProvider.java
index cd034ad..1eabb20 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/webhooks/DefaultHttpClientProvider.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/webhooks/DefaultHttpClientProvider.java
@@ -15,6 +15,7 @@
 package com.googlesource.gerrit.plugins.webhooks;
 
 import com.google.common.flogger.FluentLogger;
+import com.google.gerrit.common.Nullable;
 import com.google.inject.Inject;
 import com.google.inject.Provider;
 import java.security.KeyManagementException;
@@ -51,6 +52,7 @@
             .build();
   }
 
+  @Nullable
   private static SSLContext buildSslContext() {
     try {
       TrustManager[] trustAllCerts = new TrustManager[] {new DummyX509TrustManager()};