lib: Return Null when we have an empty string for script name

lib: Return Null when we have an empty string for script name

The script entries were being stored in a hash lookup with
the script name a pre-defined array of characters.  The hash
lookup is succeeding since it is auto-installed at script
start time irrelevant if there is a handler function.

Modify the code so that if the scriptname is an empty
string "\0" just return a NULL so that zebra does
not attempt to actually load up the script

Signed-off-by: Donald Sharp <sharpd@nvidia.com>