fix(validatable): fix error-messages comparison
fix(validatable): fix error-messages comparison

if error-messages was assigned null, comparison would break

fixes #4130

vuetifyjs/vuetify,Webifi/vuetify,vuetifyjs/vuetify,Phlow2001/vuetify,Webifi/vuetify,Webifi/vuetify,Phlow2001/vuetify,Phlow2001/vuetify,vuetifyjs/vuetify,Phlow2001/vuetify,vuetifyjs/vuetify