Add comment about zero length case in batch verification

[jtraglia]

The team auditing c-kzg-4844 had this minor critique, which I agree with:

I think it may be worth the extra clarifications in the specs on how the zero length case should be handled.

This PR adds a comment to the verify_blob_kzg_proof_batch docstring about this.

Also, delete the blank line to be more in line with other docstrings. For example:

consensus-specs/specs/deneb/polynomial-commitments.md

Lines 445 to 449 in fe9c1a8

	"""
	Compute KZG proof at point `z` for the polynomial represented by `blob`.
	Do this by computing the quotient polynomial in evaluation form: q(x) = (p(x) - p(z)) / (x - z).
	Public method.
	"""

consensus-specs/specs/deneb/polynomial-commitments.md

Lines 358 to 362 in fe9c1a8

	"""
	Verify KZG proof that ``p(z) == y`` where ``p(z)`` is the polynomial represented by ``polynomial_kzg``.
	Receives inputs as bytes.
	Public method.
	"""

[asn-d6]

LGTM! Thanks!