make the default value of `first_key` hashable

[wyli]

Is your feature request related to a problem? Please describe.
the default of first_key is an empty list which is not hashable:

MONAI/monai/transforms/transform.py

Lines 415 to 424 in a3f3504

	def first_key(self, data: Dict[Hashable, Any]):
	"""
	Get the first available key of `self.keys` in the input `data` dictionary.
	If no available key, return an empty list `[]`.
	Args:
	data: data that the transform will be applied to.
	"""
	return first(self.key_iterator(data), [])

this makes the regular usage: data_dict[first_key(d)] always raises a static type checking error about not hashable.

Revising the default value might resolve this issue.