I very often use this pattern when setting a Default value on a component:
{{table.selectedRow.data.start_time === null ? '8:00' : table.selectedRow.data.start_time}}
I check to see if the source data is falsy (null, undefined, "") and if so put in a hard coded default value.
Not too difficult really, but it would be a minor quality of life improvement to have a Default if falsy.
I would set table.selectedRow.data.start_time
in the normal Default value property. And the new Default if falsy property would be set to '8:00'
which would override Default value if it were falsy.