Fix: config must be non-null but is marked as @Nullable

Change-Id: I1bf6accd0f7be07352163c235eaad330b6cde64c

gitiles-servlet/src/main/java/com/google/gitiles/GitilesServlet.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/GitilesServlet.java b/gitiles-servlet/src/main/java/com/google/gitiles/GitilesServlet.java
index 472f0a8..d015f7a 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/GitilesServlet.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/GitilesServlet.java
@@ -45,7 +45,7 @@
   public static final String STATIC_PREFIX = "/+static/";
 
   public GitilesServlet(
-      @Nullable Config config,
+      Config config,
       @Nullable Renderer renderer,
       @Nullable GitilesUrls urls,
       @Nullable GitilesAccess.Factory accessFactory,