🐛 fix: serialize Pendulum zero Duration as valid ISO 8601 'P0D'

[dreglad]

This PR fixes the serialization of zero-value Pendulum durations. Previously, serializing a zero duration produced the invalid ISO 8601 string 'P', which caused validation errors on deserialization. Now, zero durations are correctly serialized as 'P0D'.

Before

TestModel(delta_t=Duration(0)).model_dump_json()
# Output: {"delta":"P"}
# Validation error on reload

After

TestModel(delta_t=Duration(0)).model_dump_json()
# Output: {"delta":"P0D"}
# No validation error

[yezz123]

LGTM 🚀