I don't think the sys.dm_exec_procedure_stats view can return a NULL execution_count. It's definition represents that column as non-nullable.
Are you sure you're querying the view correctly? Do you still see the same results for that procedure using the following query?
SELECT DB_NAME(PS.database_id) AS DatabaseName,
OBJECT_NAME(PS.object_id, PS.database_id) AS ObjectName,
FROM sys.dm_exec_procedure_stats AS PS;
I know procedures only appear in this view if they're cached. The procedure must also finish before the view is updated. I don't think that procedures using a "WITH RECOMPILE" hint appear here in this view, either.