Define behavior of NIL for readtable argument

[yitzchak]

COPY-READTABLE states that passing NIL as the readtable argument will cause the standard readtable to be copied. The other readtable functions do not state the behavior of NIL as the readtable argument. Dictionary entries affected:

• set-macro-character, get-macro-character
• make-dispatch-macro-character
• readtable-case
• set-dispatch-macro-character, get-dispatch-macro-character
• set-syntax-from-char

GET-MACRO-CHARACTER

Some examples for just one of the functions, GET-MACRO-CHARACTER using the following test case:

(defun one ()
  (let ((func (lambda (x y)))
        (*readtable* (copy-readtable)))
    (set-macro-character #\a func *readtable*)
    (values (eq func (get-macro-character #\a))
            (eq func (get-macro-character #\a nil)))))
    
(one)

• ABCL (one) ; => T, NIL
• CCL (one) ; => T, T
• Clasp (one) ; => T, T
• ECL (one) ; => T, NIL
• SBCL (one) ; => T, NIL