UI: merge button should not show separator when no extra options

[hyrious]

vscode-pull-request-github/webviews/components/dropdown.tsx

Line 99 in 6dbe4e1

<div className={`split${disabled ? ' disabled' : ''}`}><div className={`separator${disabled ? ' disabled' : ''}`}></div></div>

Related issue: #5530

[aiday-mar]

Looks fixed although the > is not centered in the button