whitespace: Fix unintended change in buffer modification status

whitespace: Fix unintended change in buffer modification status

* lisp/whitespace.el (whitespace--empty-at-bob-matcher)
whitespace--empty-at-eob-matcher, whitespace--update-bob-eob):
Silently add the `font-lock-multiline' text property when
highlighting beginning-of-buffer and end-of-buffer empty lines
to prevent Emacs from running modification hooks or considering
the buffer to be modified (Bug#60066).

* test/lisp/whitespace-tests.el
(whitespace-tests--empty-bob-eob-modified): Add a regression test.