bug#60188: 30.0.50; zerop cpd-length are not all handled in project--read-file-cpd-relative

[Shuguang Sun via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Dear all,

When I call C-x p d (project-find-dir), it reports the error message:
`project--read-file-cpd-relative: Wrong type argument: stringp, nil`

Finaly I found it is due the the zero length of
`common-parent-directory` in `project--read-file-cpd-relative` as I
using `fd` to provide the list of dirs which does not include the common
project root in the returnd dirs.

In `project--read-file-cpd-relative`, the zero length of
`common-parent-directory` is handled for `prompt` but not for `abbr-cpd`
which will be used as parameter of `string-prefix-p`.

Below is the diff and a working patch. But no sure for the impact of HIST.

```
diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index 559da6dd649..8c4ea8eeca0 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -1039,7 +1039,9 @@ project--read-file-cpd-relative
          (_ (when included-cpd
               (setq substrings (cons "./" substrings))))
          (new-collection (project--file-completion-table substrings))
-         (abbr-cpd (abbreviate-file-name common-parent-directory))
+         (abbr-cpd (if (zerop cpd-length)
+                       ""
+                     (abbreviate-file-name common-parent-directory)))
          (relname (cl-letf ((history-add-new-input nil)
                             ((symbol-value hist)
                              (mapcan
```

Best Regards,
Shuguang Sun

In GNU Emacs 30.0.50 (build 1, x86_64-w64-mingw32) of 2022-12-18 built
 on YJ190169
Repository revision: 2c2ecb46b0ad3e841ac9551e3a80d02893cdf6ec
Repository branch: master
Windowing system distributor 'Microsoft Corp.', version 10.0.22621
System Description: Microsoft Windows 10 Home China (v10.0.2009.22621.963)

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