Fix NPE with empty rulesets.

This is the fix for bug #1155 (https://sourceforge.net/p/pmd/bugs/1155/)

The whole code is in dire need of revisiting its behaviour when a
value is null or empty. Same goes for e.g. createRules whether it
should return null values (then the caller will have to deal with it)
or empty Rulesets.

Catching and discarding the rule not found exception looks pretty
suspicious.
Merge pull request #31 from hgschmie/npe-fix

Fix NPE with empty rulesets.