isEmailAvailable returns inverted value

when using isEmailAvailable (positive) in correlation with exists (positive) make sure that the outcome is a positive return (and not an inverted). It is correct to cast '''exists''' as a boolean, but one forgot to then actually invert the value again.

!!undefined = false
!!!undefined = true

!false = true
!!false = false
!!!false = true

!true = false
!!true = true
!!!true = false