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();