Why phone encryption without screen lock is possible under Marshmallow?
I have a Galaxy S5 (Android Marshmallow), and when I hit the encrypt phone button, my phone stopped me and forced me to have a password or pin in order to continue. If you use finger print or face recognition, you will need to enter the password or pin on device restart. So the answer is Yes your phone will disallow you from continuing with phone encryption without a password or pin, your phone will just ask you nicely for a password as a part of the encryption process rather than send you back into the settings to make a password.