Howto check if variable doesn't have value in AEL

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1

I could use EXISTS() and ISNULL(), but shorter expression like following works fine for EXISTS():
if (${varname})
{
do something;
};
and I like it better than having complicating

if (${EXISTS(${varname})})

what would be equivalent for ISNULL()?

if ([color=red]![/color]${varname})
{
do something;
};