get_track should check that the track exists

[KB1RD]

Describe the bug
get_track does no checks to ensure the track exists (in the XML file), so if the track cannot be found, an error with an unclear explanation will be thrown

To Reproduce
Steps to reproduce the behavior:

from pyrekordbox.rbxml import RekordboxXml
xml = RekordboxXml()
xml.get_track(Location="/does/not/exist")
...

The resulting error is:

TypeError: SubElement() argument 1 must be xml.etree.ElementTree.Element, not None

Expected behavior
Probably should just return None if there's no track found, or raise an error. The following inserted here should fix it:

return None if el is None else Track(element=el)

Environment

• Pyrekordbox version: 0.4.4
• Rekordbox version: N/A (not installed on my current computer)

[tonarec]

Not tested yet but you may have the same error with an invalid TrackID

[KB1RD]

Yes, rekordbox doesn't load anything with a different TrackID. I've tried taking a TrackID from an existing rekordbox export and changing it by 1, and rekordbox won't load the track. Rekordbox also won't load a track with any of the default fields missing, and also requires the track length be parseable as an int (maybe float too?) regardless of whether it's actually correct.

Do you know what constitutes a valid TrackID? Been trying to figure it out.

For a given track, multiple TrackIDs will work, but I can't seem to find the pattern in them yet.