Allow an empty array for "required".

This addresses the enhancement requested in issue json-schema-org#69.

Author: handrews

jsonschema-validation.xml

@@ -397,9 +397,8 @@
 
             <section title="required">
                 <t>
-                    The value of this keyword MUST be an array. This array MUST have at
-                    least one element. Elements of this array MUST be strings, and MUST be
-                    unique.
+                    The value of this keyword MUST be an array. Elements of this array
+                    MUST be strings, and MUST be unique.
                 </t>
                 <t>
                     An object instance is valid against this keyword if its

schema.json

@@ -21,8 +21,10 @@
         "stringArray": {
             "type": "array",
             "items": { "type": "string" },
-            "minItems": 1,
             "uniqueItems": true
+        },
+        "nonEmptyStringArray": {
+            "allOf": [ { "$ref": "#/definitions/stringArray" }, { "minItems": 1 } ]
         }
     },
     "type": "object",
@@ -117,7 +119,7 @@
             "additionalProperties": {
                 "anyOf": [
                     { "$ref": "#" },
-                    { "$ref": "#/definitions/stringArray" }
+                    { "$ref": "#/definitions/nonEmptyStringArray" }
                 ]
             }
         },