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#
1 file changed