[Bug]: `ET.parse(orbit_path)` FileNotFoundError

Checked for duplicates

Yes - I've already checked. Found by Ozzy Tirmizi.

Describe the bug

>>> bursts = s1reader.load_bursts(zip_path, orbit_path, swath_num=2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/yunjunz/tools/s1-reader/src/s1reader/s1_reader.py", line 476, in load_bursts
    bursts = _burst_from_zip(path, id_str, orbit_path)
  File "/Users/yunjunz/tools/s1-reader/src/s1reader/s1_reader.py", line 536, in _burst_from_zip
    bursts = burst_from_xml(f_annotation, orbit_path, f_tiff, iw2_f_annotation, z_file.open)
  File "/Users/yunjunz/tools/s1-reader/src/s1reader/s1_reader.py", line 339, in burst_from_xml
    orbit_tree = ET.parse(orbit_path)
  File "/Users/yunjunz/tools/miniconda3/envs/opera/lib/python3.10/xml/etree/ElementTree.py", line 1229, in parse
    tree.parse(source, parser)
  File "/Users/yunjunz/tools/miniconda3/envs/opera/lib/python3.10/xml/etree/ElementTree.py", line 569, in parse
    source = open(source, "rb")
FileNotFoundError: [Errno 2] No such file or directory: ''
>>> 

What did you expect?

I expected [...]

Reproducible steps

import s1reader
zip_file = './SLC/S1A_IW_SLC__1SSV_20160829T230628_20160829T230655_012821_014394_A1F0.zip'
orbit_dir = './orbits'
orbit_path = s1reader.get_orbit_file_from_dir(zip_path, orbit_dir)
bursts = s1reader.load_bursts(zip_path, orbit_path, swath_num=2)

Environment

- Version of this software [e.g. vX.Y.Z]
- Operating System: [e.g. MacOSX with Docker Desktop vX.Y]
...

Reproduce the call in the issue, then inspect src/s1reader/s1_reader.py at burst_from_xml (line 339), _burst_from_zip (line 536), and load_bursts (line 476), along with get_orbit_file_from_dir. Confirm why orbit_path is empty; done means load_bursts receives a valid orbit file and no longer raises FileNotFoundError.