Some of Debian's test runners noticed that the pydantic-extra-types
tests are failing on 32-bit architectures:

  ______________________ test_invalid_zero_duration_string _______________________

      def test_invalid_zero_duration_string():
          """'P' is not a valid ISO 8601 duration and should raise a validation error."""
  >       with pytest.raises(ValidationError):
  E       Failed: DID NOT RAISE <class 'pydantic_core._pydantic_core.ValidationError'>

  tests/test_pendulum_dt.py:447: Failed

Debian currently has pendulum 3.0.0, which disabled the Rust extensions
if `struct.calcsize("P") == 4`, and the Rust and Python parsers disagree
about how to handle an empty duration: the Rust parser reports an error,
while the Python parser returns `Duration()`.  3.1.0 removes that
particular limitation on using Rust extensions on 32-bit architectures,
but the parser discrepancy still seems to be present.

I don't have access to the full text of the standard, but Wikipedia's
summary says 'However, at least one element must be present, thus "P" is
not a valid representation for a duration of 0 seconds', so I think the
Rust parser is correct.  Adjust the Python parser to match.