In the following context taken from the mIRC help file:
You can use the ! prefix in front of variables and identifiers in an if-then-else statement to negate a value. The following statements are equivalent.
a) if (%x == $null) echo x has no value
b) if (!%x) echo x has no value
i can say it's false and let me explain why!
1. If %x doesn't exist, than the both points are true!
2. if you set %x to 0 than both a) and b) will not be equivalent, because %x is not null and ! negate the %x. The same if we set %x to $false.
Anyway, a) is different from b)
So, mIRC help file must be corrected