How do I check if a string value has a value assigned to it?

So I want to assign a value to my string value if no value has been assigned to it yet.

tool.AncestryChanged:Connect(function(c, p)
    if p:FindFirstChild('Humanoid') then
        if tool.PN.Value == " " then
            tool.PN.Value = p.Name
        end
    end
end)

That's all I could think of, and it doesn't work.