Merge #586: Allow an empty email field in the registration form when email is optional

96ef18e fix: [#585] email field if regsitration form can be empty (Jose Celano)

Pull request description:

  Depends on: torrust/torrust-index#670

  Allow an empty email field in the registration form when email is optional in the configuration.

  To be fully fixed this [PR](torrust/torrust-index#670) has to be merged.

ACKs for top commit:
  josecelano:
    ACK 96ef18e

Tree-SHA512: d9f18e7acceeac8b8016ac8cf12fae5240b78e5f610c7ad522a56f5a5046e95e5b1b50d6c682f43a94cb2c0572968da45d276ca517b1c7e63df33a6ecda7b183

Author: josecelano

components/registration/RegistrationForm.vue

@@ -12,7 +12,7 @@
         <template v-if="settings && settings.email_on_signup !== EmailOnSignup.Ignored">
           <FormInputText
             v-model="form.email"
-            type="email"
+            :type="settings.email_on_signup === EmailOnSignup.Required || form.email !== '' ? 'email' : 'text'"
             label="Email"
             name="email"
             data-cy="registration-form-email"