fix(schema): handle required: null and required: undefined as required: false

Fix #8219

Author: vkarpov15

lib/schematype.js

@@ -798,6 +798,17 @@ const handleIsAsync = util.deprecate(function handleIsAsync() {},
 
 SchemaType.prototype.required = function(required, message) {
   let customOptions = {};
+
+  if (arguments.length > 0 && required == null) {
+    this.validators = this.validators.filter(function(v) {
+      return v.validator !== this.requiredValidator;
+    }, this);
+
+    this.isRequired = false;
+    delete this.originalRequiredValue;
+    return this;
+  }
+
   if (typeof required === 'object') {
     customOptions = required;
     message = customOptions.message || message;