LYNX-198: Set is_default as not nullable in schema

Author: svera

app/code/Magento/EavGraphQl/Model/Output/GetAttributeData.php

@@ -108,8 +108,7 @@ function (AttributeOptionInterface $option) use ($attribute) {
                     return [
                         'label' => $label,
                         'value' => $value,
-                        'is_default' => $attribute->getDefaultValue() ?
-                            in_array($value, explode(',', $attribute->getDefaultValue())) : null
+                        'is_default' => in_array($value, explode(',', $attribute->getDefaultValue()))
                     ];
                 },
                 $attribute->getOptions()

app/code/Magento/EavGraphQl/etc/schema.graphqls

@@ -91,7 +91,7 @@ interface CustomAttributeMetadataInterface @typeResolver(class: "Magento\\EavGra
 interface CustomAttributeOptionInterface @typeResolver(class: "Magento\\EavGraphQl\\Model\\TypeResolver\\AttributeOption") {
     label: String! @doc(description: "The label assigned to the attribute option.")
     value: String! @doc(description: "The attribute option value.")
-    is_default: Boolean @doc(description: "Is the option value default.")
+    is_default: Boolean! @doc(description: "Is the option value default.")
 }
 
 type AttributeOptionMetadata implements CustomAttributeOptionInterface @doc(description: "Base EAV implementation of CustomAttributeOptionInterface.") {