[chore] - check otp_auth login case statement

[ErikFerrari]

Check if using the following data the server return 400 instead of 500.
{"username":"foo@bar.it","password":"Somepassword","otp": null}

Other test:

• valid email and password but null otp
• valid email and password but invalid otp
• invalid email but right password and null otp
• invalid email but right password and valid otp
• invalid email but right password and invalid otp
• invalid password but right email and null otp
• invalid password but right email and valid otp
• invalid password but right email and invalid otp