bug#67226: 30.0.50; Regression in previous-complete-history-element with empty input

[Juri Linkov]

> When using previous-complete-history-element like:
>
> The behavior changed sometime recently, because
>
> When the input is empty it shows:
>
>  [No earlier matching history item]
>
> Previously it behaved like `previous-history-element` in that case.
>
> Is this intended due to some recent change or is it an issue?
>
> ```
> Emacs -Q
>
> M-: (keymap-set minibuffer-local-map "<up>" 
> #'previous-complete-history-element)
>
> M-x some-command1
> M-x some-command2
> M-x some-command3
> M-x <up-arrow>
> ```

The regression is on this line in `previous-matching-history-element`:

  (string-match (concat ".*\\(" regexp "\\)") match-string)

In GNU Emacs 29.1.90

  (string-match ".*\\(^\\)" "foo")
  => 0

But in GNU Emacs 30.0.50

  (string-match ".*\\(^\\)" "foo")
  => nil

Cc:ing Stefan.