let lib handle missing src path

Changing default src from "" to null as recent library
commit[1] means that an earlier fs commit[2], which worked
around lib shortcoming, can be reverted.

[1] https://code.google.com/p/plexi/source/detail?r=05892ca8b0e8b7146da2142a48fb83d4619911a0
[2] https://code.google.com/p/plexi/source/detail?r=4ef8f07341a562a4c2e2bebb5e1007ba2f56b5d9&repo=fs&path=/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 efd8fae..977f924 100644
--- a/src/com/google/enterprise/adaptor/fs/FsAdaptor.java
+++ b/src/com/google/enterprise/adaptor/fs/FsAdaptor.java
@@ -206,7 +206,7 @@
 
   @Override
   public void initConfig(Config config) {
-    config.addKey(CONFIG_SRC, "");
+    config.addKey(CONFIG_SRC, null);
     config.addKey(CONFIG_SUPPORTED_ACCOUNTS,
         "BUILTIN\\Administrators,Everyone,BUILTIN\\Users,BUILTIN\\Guest,"
         + "NT AUTHORITY\\INTERACTIVE,NT AUTHORITY\\Authenticated Users");