fix: handle null pointer in to_string util

Author: MunifTanjim

evdev/util.lua

@@ -48,7 +48,11 @@ end
 ---@param str_ptr ffi.cdata* pointer to string
 ---@return string
 function mod.to_string(str_ptr)
-  return str_ptr and ffi.string(str_ptr) or ""
+  if str_ptr == nil then
+    return ""
+  end
+
+  return ffi.string(str_ptr)
 end
 
 ---@param pathname string