mruby-print/print.rb: reduce explicit return values; ref #5762

By making __printstr__ to return nil.

mrbgems/mruby-print/mrblib/print.rb

@@ -47,11 +47,9 @@ module Kernel
       i += 1
     end
     __printstr__ "\n" if len == 0
-    nil
   end
 
   def printf(*args)
     __printstr__(sprintf(*args))
-    nil
   end
 end

mrbgems/mruby-print/src/print.c

@@ -43,7 +43,7 @@ mrb_printstr(mrb_state *mrb, mrb_value self)
   if (mrb_string_p(s)) {
     printstr(mrb, RSTRING_PTR(s), RSTRING_LEN(s));
   }
-  return s;
+  return mrb_nil_value();
 }
 
 void