locate: Accept ‘--clear’ without additional arguments.

Fixes a bug whereby ‘guix locate --clear’ would end with the “no files
to search for” error.

Fixes <https://issues.guix.gnu.org/66799>.

* guix/scripts/locate.scm (guix-locate): Do not emit “no files to search
for” error when 'clear? is set in OPTS.
* tests/guix-locate.sh: Test it.

Reported-by: Maciej Kalandyk <m.kalandyk@outlook.com>
Change-Id: Ib8fa125c18481d7f5408bd89df9503713527641d

guix/scripts/locate.scm

@@ -657,7 +657,7 @@ Locate FILE and return the list of packages that contain it.\n"))
                                  files)))
             (()
              (if (null? files)
-                 (unless update?
+                 (unless (or update? (assoc-ref opts 'clear?))
                    (leave (G_ "no files to search for~%")))
                  (leave (N_ "file~{ '~a'~} not found in database '~a'~%"
                             "files~{ '~a'~} not found in database '~a'~%"

tests/guix-locate.sh

@@ -70,3 +70,9 @@ then
     $cmd_store guile | grep "$(guix build guile-bootstrap)/bin/guile"
     $cmd_store boot-9.scm | grep ^guile-bootstrap
 fi
+
+# The command below is an error: "no files to search for"...
+guix locate && false
+
+# ... but this one is fine.
+guix locate --clear