fix(ipv4): TS option maker rejects its own default timestamp=None and prints it without !r #2277

Describe the bug
IPv4._make_opt_ts accepts only a list or a dict for timestamp, but its default is None. So a TS option built without timestamps always falls through to the error branch. That branch formats with {timestamp} rather than {timestamp!r} (#2232, #2257), so 'abc' renders as a bare word, and the docstring's Raises: lists only FieldValueError.

Reproduction

IPv4(src='1.2.3.4', dst='5.6.7.8', options=[(OptionNumber.TS, {})])
# ProtocolError: IPv4: [OptNo 68] invalid timestamp value: None
IPv4.__new__(IPv4)._make_opt_ts(OptionNumber.TS, timestamp='abc')
# ProtocolError: IPv4: [OptNo 68] invalid timestamp value: abc

timestamp=[] builds: length 24, pointer 5.

Expected behavior
timestamp=None builds like [], a Timestamp-only option with counts empty slots. Anything else that is not a list or a dict raises IPv4: [OptNo 68] invalid timestamp value: must be a list or a dict, not 'abc'. Suggested fix: treat None as [] before the type dispatch, use !r, and add ProtocolError to Raises:. test_ipv4_unit asserts assertRaises(ProtocolError) for timestamp=None, so that case flips; no test pins the text.

System information
Linux, CPython 3.14.9, pcapkit 1.5.0b14 at main 9d504ead5.