spatial_filter.py: use nan-ignoring filter for `highpass_gaussian`

[scottstanie]

referenced here https://github.com/insarlab/MintPy/pull/1018#issuecomment-1595882673

Description of proposed changes

Reminders

• [x] Pass Pre-commit check (green)
• [x] Pass Codacy code review (green)
• [x] Pass Circle CI test (green)
• [x] Make sure that your code follows our style. Use the other functions/files as a basis.
• [x] If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.