refactor(callback): warn if callback function is nil/false

Between the headache of people misusing callbacks
and people complaining about third-party resources
that misused them, I would rather just shut them up.

I'll just deprecate this crap at some point anyway.

imports/callback/client.lua

@@ -68,9 +68,14 @@ end
 ---@overload fun(event: string, delay: number | false, cb: function, ...)
 lib.callback = setmetatable({}, {
     __call = function(_, event, delay, cb, ...)
-        local cbType = type(cb)
+        if not cb then
+            warn(("callback event '%s' does not have a function to callback to and will instead await\nuse lib.callback.await or a regular event to remove this warning")
+                :format(event))
+        else
+            local cbType = type(cb)
 
-        assert(cbType == 'function', ("expected argument 3 to have type 'function' (received %s)"):format(cbType))
+            assert(cbType == 'function', ("expected argument 3 to have type 'function' (received %s)"):format(cbType))
+        end
 
         return triggerServerCallback(_, event, delay, cb, ...)
     end

imports/callback/server.lua

@@ -51,9 +51,14 @@ end
 ---@overload fun(event: string, playerId: number, cb: function, ...)
 lib.callback = setmetatable({}, {
     __call = function(_, event, playerId, cb, ...)
-        local cbType = type(cb)
+        if not cb then
+            warn(("callback event '%s' does not have a function to callback to and will instead await\nuse lib.callback.await or a regular event to remove this warning")
+                :format(event))
+        else
+            local cbType = type(cb)
 
-        assert(cbType == 'function', ("expected argument 3 to have type 'function' (received %s)"):format(cbType))
+            assert(cbType == 'function', ("expected argument 3 to have type 'function' (received %s)"):format(cbType))
+        end
 
         return triggerClientCallback(_, event, playerId, cb, ...)
     end