Trigger prototype without host:key references is saved in database

Frontend allows user to successfully save trigger prototype with expression that does not contain any host:item references (for example "0=0"). Such trigger does not show up in frontend in connection to any host or template, obviously, but still can be seen in database (with expression "{}0=0").

In similar situation with triggers we show error message:

Cannot add trigger
Details: Trigger expression must contain at least one host:key reference.