How To Use the Null Property
When certain atomic expressions (variable name, field name, form field name, or formal parameter name) are used before being assigned values, or if the values are unknown, those expressions are considered to be $Null.
$Null is a property indicating a state of valuelessness; $Null is not itself a value. The null string is a value and is therefore not the same as the $Null property.
An arithmetic or functional expression that includes a $Null argument or operand always evaluates to $Null.
A logic expression that includes a $Null operand always evaluates to $False.
Because $Null is not a value, you cannot use the conditional operators = (Equals) and <> (Not Equals) to determine if an expression is $Null. Instead, the operator IS [NOT] [$]NULL is used as shown in the following example:
Salary is $null
Salary is not $null
Note: $Null can be written without the dollar sign.