Update valid prop on MultiCheckboxInput to return true if the field is optional

js/components/multi_checkbox_input.js

@@ -14,6 +14,7 @@ export default {
       default: () => [],
     },
     initialOtherValue: String,
+    optional: Boolean,
   },
 
   data: function() {
@@ -45,7 +46,7 @@ export default {
 
   computed: {
     valid: function() {
-      return this.showValid
+      return this.optional || this.showValid
     },
   },
 }

templates/components/multi_checkbox_input.html

@@ -11,6 +11,7 @@
     {% if other_input_field and other_input_field.data and other_input_field.data != "None" %}
       initial-other-value="{{ other_input_field.data }}"
     {% endif %}
+    v-bind:optional={{ optional|lower }}
     key='{{ field.name }}'>
     <div
       v-bind:class="['usa-input', { 'usa-input--error': showError, 'usa-input--success': showValid }]">