fix(GlCheckbox): always add a label, even for empty content

mentioned in commit 430fbb5e

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!55 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!352 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!172328 (closed)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1249 (closed)

mentioned in commit 5801a20c

mentioned in commit 3c3c697e

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!58 (merged)