@PartId should probably be declared as an Int and not a VarChar

@PartId is not initialzed and therefore is null before the select:

If the select returns 0 rows then the @PartId will be unchanged and will remain null.

If the select does contain rows and the all values for Part_ID are null then @PartId will be assigned null.

After the select:

    Set @PartId = IsNull( @PartId, 1 )

Whether the select returns a null or returns no rows, the Set IsNull will give the desired results.