in https://www.zabbix.com/documentation/3.0/manual/config/items/itemtypes/zabbix_agent, it says for web.page.get:
"Returns an empty string on fail."
for web.page.regexp it says :
"Returns an empty string if no match was found."
if the regexp item times out (and probably in other failure cases), it also returns an empty string - that might be worth mentioning.