ci: make io_print accept nil as input argument (#2445)

Convert nil to an empty string, which mimicks the behavior of standard
print

scripts/check-queries.lua

@@ -3,6 +3,9 @@
 -- Equivalent to print(), but this will ensure consistent output regardless of
 -- operating system.
 local function io_print(text)
+  if not text then
+    text = ""
+  end
   io.write(text, "\n")
 end