Allow null paths in GitwebRedirectFilter

URLs such as the following are valid, and implicitly point to the root
tree:
http://git.kernel.org/?p=git/git.git;a=tree

Change-Id: Ice626a1787148288c6bfb182c06fa9d8a0f5a99c

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

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/GitwebRedirectFilter.java b/gitiles-servlet/src/main/java/com/google/gitiles/GitwebRedirectFilter.java
index ff1718b..8942438 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/GitwebRedirectFilter.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/GitwebRedirectFilter.java
@@ -77,7 +77,7 @@
     ListMultimap<String, String> params = parse(req.getQueryString());
     String action = getFirst(params, "a");
     String project = getFirst(params, "p");
-    String path = getFirst(params, "f");
+    String path = Strings.nullToEmpty(getFirst(params, "f"));
 
     // According to gitweb's perl source code, the primary parameters are these
     // short abbreviated names. When pointing to blob or subtree hash,hashParent