GREG-154: Add gender field

The field is showing when the institution is UiB.

frontend/src/routes/guest/register/steps/register.tsx

@@ -60,7 +60,7 @@ const GuestRegisterStep = forwardRef(
     const [passportNationality, setPassportNationality] = useState<
       string | undefined
     >(undefined)
-    const [gender, setGender] = useState<string | undefined>(undefined)
+    const [gender, setGender] = useState<string>('')
     const [idErrorState, setIdErrorState] = useState<string>('')
 
     console.log('register step registerData', registerData)
@@ -161,7 +161,7 @@ const GuestRegisterStep = forwardRef(
         setGender(event.target.value)
         setValue('gender', event.target.value)
       } else {
-        setGender(undefined)
+        setGender('')
         setValue('gender', null)
       }
     }