Fix none birthday showing on profile

Ghost User
2019-05-15T19:51:59+02:00

Closes #868

#### Previous behaviour
Steps to reproduce:

1. Make sure you find a user without a value for the birthday
2. Go to the profile
3. Check the value shown at the birthday, it should be `None`

#### New behaviour
Steps to validate that it works:

1. Make sure you find a user without a value for the birthday
2. Go to the profile
3. Check that the birthday is hidden