lint: archival: Fix crash in non-Git case.

Fixes a bug introduced in 29f3089c84 where
‘guix lint -c archival guile-wisp’ (for instance) would crash with a
match error because ‘lookup-by-nar-hash’ returns a string.

* guix/lint.scm (check-archival): Add SWHID case in the non-Git case.

Change-Id: I66fb060172d372041df47d90a14df168b0fa762d

guix/lint.scm

@@ -1736,6 +1736,8 @@ Disarchive entry refers to non-existent SWH directory '~a'")
                                               (list id)
                                               #:field 'source)))))))
                    ((? content?)
+                    '())
+                   ((? string? swhid)
                     '())))
                '()))
           ((? local-file?)