import: gem: Deal with unavailable licensing info.

Fixes <https://bugs.gnu.org/39404>.
Reported by Seth <lee.seth@tuta.io>.

* guix/import/gem.scm (<gem>)[licenses]: Adjust for non-vector
licenses.
* tests/gem.scm (test-bar-json): Change "licenses" to 'null'.
("gem-recursive-import"): Adjust accordingly.

guix/import/gem.scm

@@ -44,7 +44,12 @@
   (version       gem-version)                     ;string
   (authors       gem-authors)                     ;string
   (licenses      gem-licenses "licenses"          ;list of strings
-                 vector->list)
+                 (lambda (licenses)
+                   ;; This is sometimes #nil (the JSON 'null' value).  Arrange
+                   ;; to always return a list.
+                   (cond ((not licenses) '())
+                         ((vector? licenses) (vector->list licenses))
+                         (else '()))))
   (info          gem-info)
   (sha256        gem-sha256 "sha"                 ;bytevector
                  base16-string->bytevector)

tests/gem.scm

@@ -55,7 +55,7 @@
       { \"name\": \"bundler\" },
     ]
   },
-  \"licenses\": [\"MIT\", \"Apache 2.0\"]
+  \"licenses\": null
 }")
 
 (define test-bundler-json
@@ -138,7 +138,7 @@
               ('synopsis "Another cool gem")
               ('description "Another cool gem")
               ('home-page "https://example.com")
-              ('license ('list 'license:expat 'license:asl2.0)))
+              ('license #f))                      ;no licensing info
             ('package
               ('name "ruby-bundler")
               ('version "1.14.2")