Make truncate a no-op when count=None and when=None

[mjkanji]

It's a common pattern to have some limit that you set to a small int for testing and None for prod jobs. streamable currently raises an error when both count and when equal None. Instead, it should be a no-op that simply returns the input stream/iterable without doing anything.

This is especially useful when working with env vars (where if the value is not set, then os.getenv returns None).

# script.py
def my_func():
  return Stream(range(1e6)).truncate(os.getenv("LIMIT")).map(some_expensive_op)

# for testing
LIMIT=10 python script.py

# for prod
python script.py

[ebonnal]

Hey @mjkanji 👋🏻

Definitely makes sense, could you review #59 which also applies your suggestion to .skip?

[mjkanji]

@ebonnal Thank you for the quick response — and #59 Looks good to me!