You will not get a token.
Rule 112.7a does not apply because that rule only deals with situations when the source of the ability has been moved / removed. Helm of the Host is the source of the ability; the creature it was equipping is not.
if the source is no longer in the zone it's expected to be in at that time, its last known information is used.
If Helm of the Host is on the battlefield not equipping anything when its ability resolves, then the "equipped creature" simply doesn't exist, so there is nothing to copy. This is the same interaction whether it wasn't equipping anything at the beginning of combat to start with, or if the equipped creature was destroyed, or if it was unequipped in response (with something like Leonin Shikari).
Rule 112.7a will allow "the equipped creature" to still be something that can be determined if the Helm is destroyed in response to the trigger. Separately, rule 608.2g will allow that creature to still be copied even if that creature has also left the battlefield. But only when rule 112.7a kicked in to allow "the equipped creature" to be defined.
608.2g If an effect requires information from the game (such as the number of creatures on the battlefield), the answer is determined only once, when the effect is applied. If the effect requires information from a specific object, including the source of the ability itself, the effect uses the current information of that object if it’s in the public zone it was expected to be in; if it’s no longer in that zone, or if the effect has moved it from a public zone to a hidden zone, the effect uses the object’s last known information. See rule 112.7a. If an ability states that an object does something, it’s the object as it exists—or as it most recently existed—that does it, not the ability.
The key here is "the effect requires information from a specific object". That "specific object" won't exist if the Helm is not equipping anything, so 608.2g won't do anything in the case you ask about. But if the Helm has been removed, 112.7a allows for "a specific object" to come into play (the creature it was equipping when it last existed).
For completeness sake, the ruling from Gatherer verifies the answer:
If the equipped creature leaves the battlefield before the triggered ability of Helm of the Host resolves, or if there is no equipped creature, no token is created. However, if Helm of the Host leaves the battlefield while its triggered ability is on the stack, a token will be created of the creature it last equipped. If that creature has also left the battlefield, its last known information is used to determine what the token looks like.
