bug#52509: 27.2; `call-process-region' won't accept nil as START argument

[LdBeth]

The docstring of `call-process-region' writes:

  START and END are normally buffer positions specifying the part of the
  buffer to send to the process.
  If START is nil, that means to use the entire buffer contents; END is
  ignored.
  If START is a string, then send that string to the process
  instead of any buffer contents; END is ignored.
  The remaining arguments are optional.
  Delete the text if fourth arg DELETE is non-nil.

To reproduce this bug, define these functions:

```
(defun this-works ()
  (call-process-region (point-min) (point-max) "/bin/cat"
                       t t))

(defun this-wont ()
  (call-process-region nil 0 "/bin/cat"
                       t t))
```

and use `M-:` to execute these two function. The second one would give
the error trace:

```
Debugger entered--Lisp error: (wrong-type-argument integer-or-marker-p nil)
  call-process-region(nil 0 "/bin/cat" t t)
  this-wont()
  eval((this-wont) t)
  eval-expression((this-wont) nil nil 127)
  funcall-interactively(eval-expression (this-wont) nil nil 127)
  call-interactively(eval-expression nil nil)
  command-execute(eval-expression)
```

While if my understanding to the docstring is right, these two should
give identical effect.

The Emacs version I'm using is 27.2, however I suspect this can also
be reproduced in master branch.

-- 
LDB