Enable checking of ALL attributes if "attr" isn't specified

If JMXTrans doesn't receive an array of attributes to check, it will
check ALL attributes.
- BUT - If yaml2jmxtrans.py doesn't receive any attr's, it returns:
attr: null

This patch checks if attr == Null, and unsets it if so.
Merge pull request #64 from tewner/checkAllAttributes

Enable checking of ALL attributes if "attr" isn't specified