GitilesConfig#defaultFile: Add assertion against null configPath

The path should never be null at this point. Add a checkNotNull to
silence false-positive warning from FindBugs.

Change-Id: I68bca24a887f3b4b300ebae5536057ec3f789c26

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

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/GitilesConfig.java b/gitiles-servlet/src/main/java/com/google/gitiles/GitilesConfig.java
index 4f7b40f..9793163 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/GitilesConfig.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/GitilesConfig.java
@@ -14,6 +14,8 @@
 
 package com.google.gitiles;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import org.eclipse.jgit.errors.ConfigInvalidException;
 import org.eclipse.jgit.lib.Config;
 import org.eclipse.jgit.storage.file.FileBasedConfig;
@@ -39,6 +41,7 @@
     if (configPath == null && filterConfig != null) {
       configPath = filterConfig.getInitParameter(FILTER_CONFIG_PARAM);
     }
+    checkNotNull(configPath);
     return new File(configPath);
   }