Risks of power chargers left pluged in
If we are talking about fire risk, it is negligible for official Apple chargers. Non-zero, but pretty close to zero. Low-cost knockoffs have a significantly higher fire and shock risk due to cheap parts and sketchy wiring; UL produced an article showing a 99% failure rate of counterfeit chargers.

If we are talking about electrical pull, it is also negligible. Less than 0.1 watts pulled when a phone is not plugged in. You could have more than 150 chargers plugged in and you'd still pull less than a 10-watt light bulb. A single charger costs less than 25 cents and produces 1.5 lbs of carbon emissions (I assumed a liberal 1kWh to calculate this) for a whole year of being plugged in without a phone attached.

It is still a waste of energy and a non-zero percent fire risk, but it's up to you whether those minuscule risks, costs, and pollution amounts are even worth the energy and time expended unplugging it when not in use.