Gameplay Effects with instant duration fail to be predicted if a duration magnitude was previously set

The Duration Magnitude property is not zeroed out if a Gameplay Effect's duration policy is changed from HasDuration to Instant. As a result, UAbilitySystemComponent::ApplyGameplayEffectSpecToSelf early outs, preventing client prediction on instant GEs because the Spec's Period property is greater than zero. 

• Create a Gameplay Effect with a HasDuration Duration Policy and set a non zero value in the Duration Magnitude property
• Compile/Save and then change the Duration policy to instant - Compile/Save
• Note that the non zero Duration Magnitude is not zeroed out and is serialized into the GE's CDO. This can be quickly verified by switching the Duration Policy back to HasDuration and observing that the duration Magnitude has been preserved.