Explicitly test against None
Explicitly test against None

lukauskas/seaborn,sauliusl/seaborn,petebachant/seaborn,lukauskas/seaborn,phobson/seaborn,anntzer/seaborn,arokem/seaborn,mwaskom/seaborn,phobson/seaborn,JWarmenhoven/seaborn,anntzer/seaborn,arokem/seaborn,mwaskom/seaborn