Fix shortcut UI failing on filtering when empty command is given

[krassowski]

References

Fixes #15687

Code changes

• fix typing in ShortcutUI.tsx (use proper types instead of any)
• correct a name of private method to follow underscore convention
• fix the issue by guarding against incorrect access to second item of one-element array created by split() on an empty string

User-facing changes

1. Filtering shortcuts works in Shortcuts UI even when an extension provides incorrect but technically allowed shortcut with empty command (see jupyterlab-vim).
2. (Command label missing) placeholder is shown when a label for a command cannot be retrieved
   • I will also open a PR against jupyterlab-vim to add a dummy command so that user does not see it as first thing in the shortcuts table (Fix empty command jupyterlab-contrib/jupyterlab-vim#133)

Backwards-incompatible changes

None

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[krassowski]

The single visual regression failure is unrelated (tracked in #15670)

[jtpio]

Thanks!

[jtpio]

@meeseeksdev please backport to 4.0.x