bulk_writer default assignment not working in Client.recursive_delete

[didi-rf]

Environment details

• OS type and version: macOS Big Sur, version 11.6.4
• Python version: Python 3.9.9
• pip version: pip 21.3.1
• google-cloud-firestore version: google-cloud-firestore==2.4.0

Steps to reproduce

1. call Client.recursive_delete with bulk_writer=None (default value)

Code example

    from google.cloud import firestore
    db = firestore.Client('myProject')
    coll = db.collection("a", "b", "c")
    coll.add({"e": "e"},"d")
    db.recursive_delete(coll)

Stack trace

Traceback (most recent call last):
  File "/Users/didi/git/test.py", line 28, in <module>
    test()
  File "/Users/didi/git/test.py", line 25, in test
    db.recursive_delete(coll)
  File "/Users/didi/.virtualenvs/test/lib/python3.9/site-packages/google/cloud/firestore_v1/client.py", line 324, in recursive_delete
    return self._recursive_delete(reference, bulk_writer, chunk_size=chunk_size,)
  File "/Users/didi/.virtualenvs/test/lib/python3.9/site-packages/google/cloud/firestore_v1/client.py", line 348, in _recursive_delete
    bulk_writer.delete(doc_snap.reference)
AttributeError: 'NoneType' object has no attribute 'delete'

Work around

    from google.cloud import firestore
    db = firestore.Client('myProject')
    coll = db.collection("a", "b", "c")
    coll.add({"e": "e"},"d")
    db.recursive_delete(coll, bulk_writer=db.bulk_writer())

[didi-rf]

Here is a PR that should fix this issue: #565
Thanks!