Stages/tuned: require profile to be non-empty string

The stage schema previously accepted an empty string as a profile name.
This would not work in practice, therefore extend the schema to require
non-empty strings.

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

stages/org.osbuild.tuned.meta.json

@@ -19,7 +19,8 @@
         "description": "TuneD profile to activate. If multiple profiles are provided, TuneD will try to merge them.",
         "minItems": 1,
         "items": {
-          "type": "string"
+          "type": "string",
+          "minLength": 1
         }
       }
     }