elisp/ui-package: Fix an error on package name read

This is a followup to commit fbc2bbc.

Reported by Nicolò Balzarotti at
<#45>
and by Christopher Howard at
<https://lists.gnu.org/archive/html/bug-guix/2021-06/msg00036.html>.

* elisp/guix-ui-package.el (guix-packages-by-name): Make sure
'thing-at-point' returns a string (it can also return nil).  Otherwise,
do not read package names (as it would be a useless function call).

elisp/guix-ui-package.el

@@ -1512,16 +1512,16 @@ a version number.  Examples: \"guile\", \"[email protected]\".
 If PROFILE is nil, use `guix-current-profile'.
 Interactively with prefix, prompt for PROFILE."
   (interactive
-   (let ((packages (guix-package-names))
-         (at-point (car (split-string (thing-at-point 'symbol t)
-                                      "@"))))
-     (list (guix-read-package-name "Package: "
-                                   (and (member at-point packages)
-                                        at-point))
+   (let ((at-point (thing-at-point 'symbol t))
+         default-pkg)
+     (when (stringp at-point)
+       (let ((at-point (car (split-string at-point "@"))))
+         (setq default-pkg (and (member at-point (guix-package-names))
+                            at-point))))
+     (list (guix-read-package-name "Package: " default-pkg)
            (guix-ui-read-package-profile))))
   (guix-package-get-display profile 'name name))
 
-
 ;;;###autoload
 (defun guix-packages-by-regexp (regexp &optional params profile)
   "Search for Guix packages by REGEXP.