aptsources deb822 Section constructor fails on valid .sources file

[Mike Vastola]

When attempting to parse (what I believe is) a perfectly valid deb822-formated `.sources` file, the library's `_deb822.Section` constructor parses my file incorrectly, resulting in an exception being thrown.

At a low level, the error is `ValueError: Unable to parse section data` and is thrown in the `_deb822.Section` constructor, located at line 41 of `aptsources/_deb822.py`. The error was caused by the evaluation of `apt_pkg.TagSection(trimmed_section)` on that line with `trimmed_section` being an empty string.

At a high level, based on a cursory debug, it seems the parser incorrectly considers the first three lines of my `.sources` file -- which are, in fact, comments -- to be a stanza, and is trying to instantiate an object for them, despite them having no non-ignored content.

Notably, -- supporting my conclusion -- this issue disappears entirely if I remove the first blank line in the file (located after those comments and before the first stanza).

I am attaching my .sources file for reference.

Notes:
- FWIW, I don't use this library directly; the code at issue gets invoked by the `command-not-found` package/bash handler. (The warnings therefore show up every time I mistype a command.) In any case, I traced the issue down to this package.
- My python3-apt package is version 2.5.3ubuntu1 from lunar/main.
- This error began for me upon an upgrade from jammy to lunar. This sources file was unchanged during the upgrade (except for renaming `jammy` to `lunar`, and changing `Enabled` to `no` in the `partner` stanza).

[Mike Vastola (michael-vastola)]

Also, if it helps, here's a minimal example python file, triggering the error (requires attached `00ubuntu.sources` to be in cwd)