Fix flow and TS types of babel-types is* type checks to accept null | undefined (#9539)

* babel-types is* type checks accept null | undefiend as value TS type

* Use | null | undefined instead of any, make Flow type nullable

* Also change flow types of generated src

packages/babel-types/scripts/generators/flow.js

@@ -105,7 +105,7 @@ for (const type in t.NODE_FIELDS) {
 for (let i = 0; i < t.TYPES.length; i++) {
   let decl = `declare function is${
     t.TYPES[i]
-  }(node: Object, opts?: ?Object): boolean`;
+  }(node: ?Object, opts?: ?Object): boolean`;
 
   if (t.NODE_FIELDS[t.TYPES[i]]) {
     decl += ` %checks (node instanceof ${NODE_PREFIX}${t.TYPES[i]})`;

packages/babel-types/scripts/generators/generateValidators.js

@@ -11,7 +11,7 @@ function addIsHelper(type, aliasKeys, deprecated) {
       " === nodeType";
   }
 
-  return `export function is${type}(node: Object, opts?: Object): boolean {
+  return `export function is${type}(node: ?Object, opts?: Object): boolean {
     ${deprecated || ""}
     if (!node) return false;
 

packages/babel-types/scripts/generators/typescript.js

@@ -106,7 +106,7 @@ for (const type in t.NODE_FIELDS) {
 for (let i = 0; i < t.TYPES.length; i++) {
   let decl = `export function is${
     t.TYPES[i]
-  }(node: object, opts?: object | null): `;
+  }(node: object | null | undefined, opts?: object | null): `;
 
   if (t.NODE_FIELDS[t.TYPES[i]]) {
     decl += `node is ${t.TYPES[i]};`;