When finding java and ruby absolute paths, check if result is empty

If not found, the exec method in Facter>=2.0.0 returns an empty
string instead of nil, which results in errors like these:

Could not retrieve fact='nr_java_found', resolution='<anonymous>':
undefined method `start_with?' for nil:NilClass
Merge pull request #28 from jbarbuto/check_if_which_exec_is_empty

When finding java and ruby absolute paths, check if result is empty