what does it mean if the last execution time of a stored procedure is empty
sys.dm_exec_procedure_stats is not a persisted dataset.

It only shows details for queries that currently exist in the Cache. If the SP hasn't been run for some time then the SP will be removed from the cache in favour of more frequently executed queries.

Also the Cache can be emptied by events such as a Server Reboot, and if the SP hasn't been run since the reboot then it won't be there.