Check for empty phone number on submit

Thanks @rschamp!

src/components/registration/steps.jsx

@@ -397,6 +397,14 @@ module.exports = {
                 waiting: false
             };
         },
+        onValidSubmit: function (formData, reset, invalidate) {
+            if (!formData.phone || formData.phone.national_number === '+') {
+                return invalidate({
+                    'phone': this.props.intl.formatMessage({id: 'teacherRegistration.validationPhoneNumber'})
+                });
+            }
+            return this.props.onNextStep(formData);
+        },
         render: function () {
             var formatMessage = this.props.intl.formatMessage;
             return (
@@ -410,7 +418,7 @@ module.exports = {
                                  tipContent={formatMessage({id: 'registration.nameStepTooltip'})} />
                     </p>
                     <Card>
-                        <Form onValidSubmit={this.props.onNextStep}>
+                        <Form onValidSubmit={this.onValidSubmit}>
                             <PhoneInput label={formatMessage({id: 'teacherRegistration.phoneNumber'})}
                                         name="phone"
                                         defaultCountry={this.props.defaultCountry}