fix(types): oas schema type definition (#1970)

Author: jeremyfiel

.changeset/light-lizards-play.md

@@ -0,0 +1,6 @@
+---
+"@redocly/openapi-core": patch
+"@redocly/cli": patch
+---
+
+Updated OAS3 Schema type definition to correct `type` keyword enum, removed `null`.

packages/core/src/types/oas3.ts

@@ -332,7 +332,7 @@ const Schema: NodeType = {
     required: { type: 'array', items: { type: 'string' } },
     enum: { type: 'array' },
     type: {
-      enum: ['object', 'array', 'string', 'number', 'integer', 'boolean', 'null'],
+      enum: ['object', 'array', 'string', 'number', 'integer', 'boolean'],
     },
     allOf: listOf('Schema'),
     anyOf: listOf('Schema'),