save_hdfeos5.py: use mintpy.subset for naming if attributes missing

[falkamelung]

Description of proposed changes save_hdf5eos.py occasionally fails for subsets when attributes about the subset are missing because the subset information is used for the filename creation. With this PR, instead of failing, it uses the mintpy.subset information to generate the file name.

This is needed for MiaplPy because it uses SLCs which miss some attributes. I believe I have seen this error also for running mintpy only, but I forgot details.

Reminders

• [ ] Fix #xxxx
• [ ] Pass Pre-commit check (green)
• [ ] Pass Codacy code review (green)
• [ ] Pass Circle CI test (green)
• [ ] Make sure that your code follows our style. Use the other functions/files as a basis.
• [ ] If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.
• [ ] If adding new functionality, add a detailed description to the documentation and/or an example.

[yunjunz]

Thanks @falkamelung. The general motivation sounds reasonable to me. I have a few comments on the implementation details.