ignore undefined or empty graphite_globalSuffix

When graphite_globalSuffix is set to "" (the default value), it results
in unnamed Whisper files. For example:
/opt/graphite/storage/whisper/stats/foo/bar/.wsp

Removing the global suffix from the config when empty or undefined fixes
this.
Merge pull request #5 from genome-vendor/optional_global_suffix

ignore undefined or empty graphite_globalSuffix