Set filesystemadaptor.src default to empty string rather than null.

This allows init() to throw InvalidConfigurationException if the
property was not set.

Code Review:  http://codereview.appspot.com/91460043

src/com/google/enterprise/adaptor/fs/FsAdaptor.java

diff --git a/src/com/google/enterprise/adaptor/fs/FsAdaptor.java b/src/com/google/enterprise/adaptor/fs/FsAdaptor.java
index 9b1e8de..6360ec4 100644
--- a/src/com/google/enterprise/adaptor/fs/FsAdaptor.java
+++ b/src/com/google/enterprise/adaptor/fs/FsAdaptor.java
@@ -184,7 +184,7 @@
 
   @Override
   public void initConfig(Config config) {
-    config.addKey(CONFIG_SRC, null);
+    config.addKey(CONFIG_SRC, "");
     config.addKey(CONFIG_SUPPORTED_ACCOUNTS,
         "BUILTIN\\Administrators,Everyone,BUILTIN\\Users,BUILTIN\\Guest,"
         + "NT AUTHORITY\\INTERACTIVE,NT AUTHORITY\\Authenticated Users");