emacs-27 breaks null function

[Nelson H. F. Beebe]

Some 34 years ago, I wrote a function with the code snippet

        (if (null executing-macro) ...)

and that function has been in daily use through emacs-26.  However,
with emacs-27, I get an error with this traceback

        Symbol's value as variable is void: executing-macro

The Emacs Lisp function null is defined in C in the file data.c,
and its body 

{
  if (NILP (object))
    return Qt;
  return Qnil;
}

appears not to have changed between 2009 and 2020.  I find nothing
in the ChangeLog.* files that would explain the behavior change
in emacs-27.

Nor can I readily spot some other function that could be used
to test for the prior definition of a variable.

Comments and advice are welcome!

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