[R-meta] effect size calculation with a null standard deviation

Hi everyone, 

I'm conducting a meta-analysis in ecology. I'm using Morris "dpcc1" formulas ( [ https://journals.sagepub.com/doi/10.1177/1094428106291059 | https://journals.sagepub.com/doi/10.1177/1094428106291059 ] ) to calculate the effect size and its variance. 

The effect size calculation implies a difference between post- and pre-Mean which is then divided by the pre-Standard deviation. 
I was wondering how to deal with studies which have pre-Standard deviation = 0 (leading to a division by 0) ? 
How to deal with studies which have pre-Mean = post-Mean and a pre-Standard deviation = 0 (leading to 0 divided by 0) ? If pre-Mean = post-Mean , can we consider that the effect size is null, whatever the pre-Standard deviation ? 

The variance calculation implies a division by (N - 3) (N : sample size). 
How to deal with studies which have N = 3 ( leading to a division by 0) ? 

Thank you for your help, 

Marianne