X509_REQ_get_extensions(): Return empty stack if no extensions found

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13841)

crypto/x509/x509_req.c

@@ -152,7 +152,9 @@ STACK_OF(X509_EXTENSION) *X509_REQ_get_extensions(X509_REQ *req)
         ext = X509_ATTRIBUTE_get0_type(attr, 0);
         break;
     }
-    if (!ext || (ext->type != V_ASN1_SEQUENCE))
+    if (ext == NULL) /* no extensions is not an error */
+        return sk_X509_EXTENSION_new_null();
+    if (ext->type != V_ASN1_SEQUENCE)
         return NULL;
     p = ext->value.sequence->data;
     return (STACK_OF(X509_EXTENSION) *)