fix(button): correctly detect empty slot for icon only

mentioned in commit 645dcdb9

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!183 (merged)

mentioned in merge request status-page!849 (merged)

mentioned in merge request gitlab!128113 (closed)

mentioned in merge request gitlab-docs!4145 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3432 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!186 (merged)

mentioned in merge request status-page!851 (merged)

mentioned in merge request gitlab-docs!4162 (merged)

mentioned in merge request gitlab!126562 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!197 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3508 (merged)