Reject ISO durations with empty time sections

[ZelinZhu-Richard]

pendulum.parse("PT", strict=True) returns a zero duration, and pendulum.parse("P1DT", strict=True) returns one day, although neither input has a time component after T.

Both parsers accept the separator without requiring a following hours, minutes, or seconds component. Check component presence instead of numeric value so explicit zeros such as PT0S and P1DT0S remain valid. Invalid inputs use the existing low-level ValueError and public ParserError paths.

Addresses the empty-time-section part of #833. Repeated components, long-number parsing, overflow, and duration arithmetic remain outside this change.

Tested separately with the native extension and Python parser:

• test_parse_iso8601_duration_invalid and test_parse_duration_without_time_components: reject empty time sections, including P1.5DT, where hours derived from a date fraction must not count as a time component.
• test_parse_iso8601_duration and test_parse_duration_time_components: preserve explicit zero components and valid date/time combinations.
• Each parser passed 115 focused tests and the full suite: 1,866 passed, 3 skipped. The new rejection cases failed before the fix in both parsers.
• Typing and all configured lint/format checks passed.

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

No documentation change is needed for this parser validation fix.

AI assistance: this change was developed with an AI coding tool; I reviewed and tested it.