fix(ui): Prompt validation

Author: Guillaume Chau

packages/@vue/cli-ui/src/views/ProjectCreate.vue

@@ -425,7 +425,7 @@ export default {
 
     configurationValid () {
       return this.enabledPrompts.filter(
-        p => p.value === null
+        p => p.value === null || JSON.parse(p.value) === ''
       ).length === 0
     },
 

packages/@vue/cli-ui/src/views/ProjectPluginsAdd.vue

@@ -184,7 +184,9 @@ export default {
 
   computed: {
     configurationValid () {
-      return false
+      return this.enabledPrompts.filter(
+        p => p.value === null || JSON.parse(p.value) === ''
+      ).length === 0
     },
 
     enabledPrompts () {