How to check if an associative array is empty in powershell

Arrays have Count property, and you can check if this value is 0. So the condition you would check for is

$a.Count -eq 0

That's not an associative array, it's a regular array, but the answer is the same. Use .Count and compare to 0.

An associative array is called a [hashtable] in PowerShell and its literal form uses @{} (curly braces).

@{}.Count -eq 0  # hashtable (associative array)
@().Count -eq 0  # array