start_after(None) and the like should be a no-op and shouldn't result in an error

[crwilcox]

Attempting to execute a query with a start_after(None) will fail. This should be a no-op.

client = firestore.Client()
col = client.collection('0-test-collection-0')
query = col.order_by('__name__').start_after(None)
for doc in query.stream():
    print(doc)

  File "/Users/crwilcox/scratch/python-firestore/venv/lib/python3.8/site-packages/google/cloud/firestore_v1/base_query.py", line 758, in _normalize_cursor
    if len(document_fields) > len(orders):
TypeError: object of type 'NoneType' has no len()

I'd expect start_after, start_at, end_at, end_before to manage a None field handed to them.