How can I tell the function to only work if no tool is selected?

I don't know of any built in API to do such a thing, but it is quite easy to make a function that checks for this.

When a tool is equipped, it is transferred from the Player's backpack into the Player's Character. Knowing this, we can loop through the children of the character in order to determine if a tool is equipped:

function isToolEquipped(player)
    local character = player.Character
    if character then
        for _, child in next, character:GetChildren() do
            if child:IsA("Tool") then
                return true
            end
        end
    end
    return false -- If it hasn't returned yet, no tool is equipped
end

So in your case, you could use this in an if statement like:

if not isToolEquipped(player) then

Assuming 'player' is defined

EDIT: Hopperbin Version

function isToolEquipped(player)
    local character = player.Character
    if character then
        for _, child in next, character:GetChildren() do
            if child:IsA("Tool") then
                return true
            end
        end
    end

    for _, child in next, player.Backpack:GetChildren() do
        if child:IsA("HopperBin") and child.Active then
            return true
        end
    end

    return false -- If it hasn't returned yet, no tool is equipped
end