Should the Data Source picker accept blank labels?

[AMoo-Miki]

Describe the bug
I noticed that a test allows for the data source selection trigger, DataSourceMenuPopoverButton, to have no label and render as such. Is this is a permitted possibility?

https://github.com/opensearch-project/OpenSearch-Dashboards/tree/b9ac31e323be11d8b995945622faf371c4ff9893/src/plugins/data_source_management/public/components/popover_button/popover_button.test.tsx#L31

If it is permitted, we should use OuiButtonIcon, but if not, we should disallow it.

CC: @kgcreative