inputs/tree: enforce constraints for plain refs

When using "plain refs", that is, when using an array of strings,
we did not enforce the constraints of exactly one reference. This
was done for dictionary references.

inputs/org.osbuild.tree

@@ -30,6 +30,8 @@ SCHEMA = """
     "oneOf": [{
       "type": "array",
       "additionalItems": false,
+      "minItems": 1,
+      "maxItems": 1,
       "items": [{
         "type": "string"
       }]