Fix installation of packages without architecture field

Refactoring done in commit 58e7276b69be105735eccb3c18c2b28d3a2ec2e5
introduced a regression: a package without an architecture field
can no longer be installed without --force-architecture.

src/processarc.c

@@ -282,7 +282,8 @@ void process_archive(const char *filename) {
     return;
   }
 
-  if (strcmp(pkg->available.arch, "all") &&
+  if (strcmp(pkg->available.arch, "") &&
+      strcmp(pkg->available.arch, "all") &&
       strcmp(pkg->available.arch, native_arch))
     forcibleerr(fc_architecture,
                 _("package architecture (%s) does not match system (%s)"),