Handle empty arrays in pure Python API

[karlhillx]

Fixes #164.

The pure-Python sgp4_array() path now preallocates output arrays before looping, so zero-length inputs return the same shapes and dtypes as the accelerated implementation.

Also adds regression coverage for Satrec.sgp4_array() and SatrecArray.sgp4().

Tested with:

• python3 -m unittest sgp4.tests

[Copilot]

Pull request overview

This PR addresses issue #164 by making the pure-Python sgp4_array() / SatrecArray.sgp4() implementations return correctly shaped/dtyped NumPy outputs for zero-length time inputs, matching the accelerated (C++) behavior. It also adds regression tests to lock in the expected behavior for empty arrays.

Changes:

• Preallocate e, r, and v arrays in the pure-Python Satrec.sgp4_array() and fill them in-place to support zero-length inputs.
• Preallocate e, r, and v arrays in the pure-Python SatrecArray.sgp4() for consistent output shapes with empty inputs.
• Add unit tests asserting shapes and dtypes for empty-array inputs for both APIs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
sgp4/model.py	Updates pure-Python array APIs to preallocate outputs so empty inputs return correct shapes/dtypes.
sgp4/tests.py	Adds regression tests validating shapes/dtypes for empty-array inputs in both Satrec and SatrecArray array APIs.

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