Jenkinsfile: Fix minimal ospack building

Bulld the minimal ospack again by properly handling `null` debos arguments.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

Jenkinsfile

@@ -525,6 +525,7 @@ buildCandidates.each { name, arch ->
     arch.types.each { type, params ->
         /* merge the per-arch default with per-type params */
         def merged = [:] << arch << params
+        merged.args = merged.args ?: ""
 
         if (!params.requires) {
             /* first, build all jobs which don’t have any dependencies, in parallel */