Add check for prefix_count for older NetBox versions in nb_inventory (#696)

Author: rodvand

plugins/inventory/nb_inventory.py

@@ -876,7 +876,7 @@ def refresh_sites_lookup(self):
         self.sites_with_prefixes = set()
 
         for site in sites:
-            if site["prefix_count"] > 0:
+            if site["prefix_count"] and site["prefix_count"] > 0:
                 self.sites_with_prefixes.add(site["slug"])
                 # Used by refresh_prefixes()