PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)x
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Current behavior: when registering a duplicated shortcut, the observable returned by the duplicate shortcut registration immediately emits null on subscribe, resulting in the shortcut's callback being executed immediately even though the shortcut wasn't triggered.

Issue Number: 72

What is the new behavior?

The observable returned by the duplicated shortcut's registration is EMPTY, it never emits and completes immediately. This fixes the issue.

Does this PR introduce a breaking change?

[x] Yes
[] No

If a consumer relies on the observable emitting null to detect duplicate shortcuts and trigger custom logic, then this change is breaking.
It can be mitigated though: since rxjs EMPTY is a constant, one can check equality of the value returned by addShortcut and addSequenceShortcut to detect duplicated shortcuts:

  const shortcutObs = this.hotkeysService.addShortcut({ keys: 'control.c' });
  if (shortcutObs === EMPTY) {
     // custom logic for duplicate shortcut
  }

Other information

closes #72