I did a bit of searching and couldn't find this in the documentation. Is there a way to change the text in the ComboBox dropdown when there is no data in the source collection?
You can change it for all combo boxes through localization. You can see this in action here: https://demos.telerik.com/blazor-ui/combobox/localization - just type some gibberish in the input so the filtering shows no results, then change the language and repeat.
That said, I can understand how making this customizable per component instance could be useful, so I made the following enhancement idea page in our portal where you can track the implementation of a no-data template so you can put your own content there instead: https://feedback.telerik.com/blazor/1477531-no-data-template-in-the-combobox.