train_tripletloss: set default validation dir as NULL or there's be a crash is not set during training.
train_tripletloss: set default validation dir as NULL
or there's be a crash is not set during training.

Signed-off-by: Allen Liu <92fd0d37e138bf650113cb122302a58759669375@intel.com>

davidsandberg/facenet,davidsandberg/facenet