Allow Section.from_string to handle None

[teald]

Summary

Adds handling for None input to Section.from_string. Minimal example:

from astrodata.utils import Section

from pathlib import Path

file_path = Path("/file/that/does/not/exist.txt")  # In reality, some FITS stuff
string = file_path.read_text() if file_path.exists() else None
section = Section.from_string(string)  # Previously raised exception

This is what DRAGONS expects, and was previously implemented there.

This also adds relevant information to the Section.from_string docstring.