Simplify MIDI code - #1449

Summary

Simplify code in NAudio.Midi:

• Replace Array.Empty<T>() with equivalent []
• Use ArgumentNullException.ThrowIfNull
• Collapse loops and conditions where possible
• Use byte strings (""u8) instead of Encoding.UTF8.GetBytes
• Use static instance of MidiEventComparer instead of allocating it for each use
• Other small improvements

Release notes

• I've added a one-line bullet to the ### Unreleased section of RELEASE_NOTES.md
• This PR doesn't need a release-notes entry (internal refactor, test-only, or docs fix)
• Leave the release-notes entry for the maintainer to write

Testing

MIDI-related test passing:

• NAudio.Core.Tests.Midi
• NAudio.Sampler.Tests.LiveMidiInstrumentTests
• NAudio.Sampler.Tests.MidiFileRenderTests