Strength Charm + DRE

[Polksort]

If a Pokemon with double rainbow energy and strength charm attached to it attacks with an attack that normally does 10 damage, does strength charm apply and increase the damage (which would be 0 otherwise,) or does strength charm not take effect because it only adds damage to attacks that deal damage?

Example - Prinplup using Aqua Shower with double rainbow and strength charm attached to it

 

[PokePop]

These are both applied in Step B of the damage calculation.
Since they are simultaneous, you resolve both before checking if the attack damage is zero.