LogServlet: Handle HEAD symref to non-existent branch

The ObjectId of a Ref can be null. If HEAD is a symbolic ref to a
non-existent branch, headRef.getObjectId() will return null. The
current LogServlet code does not handle this, causing
NullPointerException in RevWalk.

This issue is particularly common when a repo is created but nothing is
pushed yet; so a HEAD symmetry is created to point to refs/heads/main by default but lacks a commit SHA.

Treat a HEAD ref with a null ObjectId as if the ref itself were null,
returning no view instead of throwing an internal error.

Change-Id: I4fe85010db0694403770923fcdf957854e72cd34
Signed-off-by: Xing Huang <xingkhuang@google.com>

java/com/google/gitiles/LogServlet.java

diff --git a/java/com/google/gitiles/LogServlet.java b/java/com/google/gitiles/LogServlet.java
index 71f7384..368fd0b 100644
--- a/java/com/google/gitiles/LogServlet.java
+++ b/java/com/google/gitiles/LogServlet.java
@@ -191,10 +191,14 @@
     if (headRef == null) {
       return null;
     }
+    ObjectId id = headRef.getObjectId();
+    if (id == null) {
+      return null;
+    }
     try (RevWalk walk = new RevWalk(repo)) {
       return GitilesView.log()
           .copyFrom(view)
-          .setRevision(Revision.peel(Constants.HEAD, walk.parseAny(headRef.getObjectId()), walk))
+          .setRevision(Revision.peel(Constants.HEAD, walk.parseAny(id), walk))
           .build();
     }
   }