Add one more valid case (graphql#287)

Author: jferrettiboke

site/learn/Learn-Schema.md

@@ -201,6 +201,7 @@ This means that the _list itself_ can be null, but it can't have any null member
 
 ```js
 myField: null // valid
+myField: [] // valid
 myField: ['a', 'b'] // valid
 myField: ['a', null, 'b'] // error
 ```
@@ -215,6 +216,7 @@ This means that the list itself cannot be null, but it can contain null values:
 
 ```js
 myField: null // error
+myField: [] // valid
 myField: ['a', 'b'] // valid
 myField: ['a', null, 'b'] // valid
 ```