remove else in good example of G-4210 to match original code better

Author: PhilippSalvisberg

docs/4-language-usage/4-control-structures/2-case-if-decode-nvl-nvl2-coalesce/g-4210.md

@@ -37,8 +37,6 @@ begin
          my_package.do_blue();
       when constants_up.co_black then
          my_package.do_black();
-      else
-         null;
    end case;
 end;
 /