ParsePhrase("") throws "Phrase must contain at least one chord (Parameter 'sequence')" from its own empty-input guard #128

What's wrong

Keybinding/Services/KeybindingService.cs:178-183:

public Phrase ParsePhrase(string phraseString)
{
    if (string.IsNullOrWhiteSpace(phraseString))
    {
        return new Phrase([]);
    }

The Phrase constructor (MusicalTypes.cs:~318) rejects an empty sequence, so this branch can never return. Every call with null, "" or whitespace throws:

ArgumentException: Phrase must contain at least one chord (Parameter 'sequence')
   at Phrase..ctor
   at KeybindingService.ParsePhrase  KeybindingService.cs:182

Why it matters

The guard was clearly written to handle empty input in a controlled way, but the exception it produces names a parameter (sequence) that the caller never passed. A settings UI that parses a user's cleared keybinding field gets a confusing error that points inside the library. ParseChord validates its argument and throws with nameof(chordString), so the two parse methods behave differently.

Suggested fix

Pick one contract and implement it:

• Throw new ArgumentException("Phrase string cannot be empty.", nameof(phraseString)), as ParseChord does. This is the minimal fix and doesn't change the API.
• Or change the method to return Phrase? (or add TryParsePhrase) and return null for empty input.

Acceptance: ParsePhrase("") no longer throws from Phrase..ctor, and a test pins the chosen behaviour.