When you trade Pokémon with other trainers, the amount of candy you receive is based on the distance between the two Pokémon.
The amount of Candy you earn depends on the distance between where the two Pokémon were caught. The further the distance, the more Candy.
Pokémon captured by field research prior to the trading system do not contain a capture location. After the trading system was released, Pokémon captured by field research now contain the capture location
Given how the number of candies is based on distance between where the Pokémon were caught, how many candies would I receive when trading Pokémon that do not contain a capture location?
What if both Pokémon do not contain a capture location and there is no distance to calculate?