Allow nullish extends in interfaceish

Otherwise code such as

```ts
t.interfaceDeclaration(
  t.identifier('id'),
  undefined,
  undefined,
  t.objectTypeAnnotation([])
)
```

Will fail when priting with ` TypeError: unknown: Cannot read property 'length' of null`
Despite nullish values being allowed in `t.interfaceDeclaration` definitions

packages/babel-generator/src/generators/flow.ts

@@ -360,7 +360,7 @@ export function _interfaceish(
 ) {
   this.print(node.id, node);
   this.print(node.typeParameters, node);
-  if (node.extends.length) {
+  if (node.extends && node.extends.length) {
     this.space();
     this.word("extends");
     this.space();

packages/babel-generator/test/index.js

@@ -462,6 +462,17 @@ describe("programmatic generation", function () {
 }`);
   });
 
+  it("flow interface with nullish extends", () => {
+    const interfaceDeclaration = t.interfaceDeclaration(
+      t.identifier("A"),
+      undefined,
+      undefined,
+      t.objectTypeAnnotation([]),
+    );
+    const output = generate(interfaceDeclaration).code;
+    expect(output).toBe("interface A {}");
+  });
+
   describe("directives", function () {
     it("preserves escapes", function () {
       const directive = t.directive(