message: Change lookupNamespaceURI method to return also null.
diff: },
  "interfaces": {
  "interface": {
  + "XPathNSResolver": {
  + "methods": {
  + "method": {
  + "lookupNamespaceURI": {
  + "override-signatures": [
  + "lookupNamespaceURI(prefix: string): string | null"
  + ]
  + }
  + }
  + }
  + },
  "CryptoKey": {
  "properties": {
  "property": {
  