Use `password.present?` in DatabaseAuthenticatable strategy

In order to be more clear about the expectations of for authenticating, we use
`password.present?` so there is no confusion about the role of the `valid_password?`
method.

More info: https://github.com/plataformatec/devise/issues/3519

lib/devise/strategies/database_authenticatable.rb

@@ -5,7 +5,7 @@ module Devise
     # Default strategy for signing in a user, based on their email and password in the database.
     class DatabaseAuthenticatable < Authenticatable
       def authenticate!
-        resource  = valid_password? && mapping.to.find_for_database_authentication(authentication_hash)
+        resource  = password.present? && mapping.to.find_for_database_authentication(authentication_hash)
         encrypted = false
 
         if validate(resource){ encrypted = true; resource.valid_password?(password) }