feat(@ngtools/json-schema): support null data type

Author: clydin

Diff for: packages/@ngtools/json-schema/src/schema-tree.ts

@@ -164,6 +164,7 @@ export abstract class NonLeafSchemaTreeNode<T> extends SchemaTreeNode<T> {
       case 'boolean': Klass = BooleanSchemaTreeNode; break;
       case 'number': Klass = NumberSchemaTreeNode; break;
       case 'integer': Klass = IntegerSchemaTreeNode; break;
+      case 'null': Klass = NullSchemaTreeNode; break;
 
       case 'enum': Klass = EnumSchemaTreeNode; break;
       case 'oneOf': Klass = OneOfSchemaTreeNode; break;
@@ -462,6 +463,13 @@ class StringSchemaTreeNode extends LeafSchemaTreeNode<string> {
   get tsType() { return String; }
 }
 
+class NullSchemaTreeNode extends LeafSchemaTreeNode<string> {
+  isCompatible(v: any) { return v == undefined || v == null; }
+  convert(_v: any): null { return null; }
+  get type() { return 'null'; }
+  get tsType() { return Object; }
+}
+
 
 class EnumSchemaTreeNode extends LeafSchemaTreeNode<any> {
   constructor(metaData: TreeNodeConstructorArgument<any>) {