Fire bolt? does not have an attack roll and will always hit.
The way this cantrip works, is that the target of the cantrip has to do a DC 20 Saving throw that is not affected any bonuses or penalties. Most of the time the target will be hit by a normal fire bolt, but if the target succeeds the saving throw they will instead be hit by a fireball.
This means that targeting something that can not roll for saving throws like the ground or an object, will always shoot a normal fire bolt.
If the target succeeds the saving throw the combat log will not only show a compeletly wrong calculation but it will also say that the target failed it.
cantrip, but has a 1/20 chance to cast 
instead for free.
Range: 18 m / 60 ft
Saving throw that is not affected any bonuses or penalties. Most of the time the target will be hit by a normal fire bolt, but if the target succeeds the saving throw they will instead be hit by a fireball.
Fire bolt on the Forgotten Realms Wiki