Load dynamic sites when there is no criteria (#83)

When a dynamic site has not filtering criteria, the AJAX call not
returns an empty criteria array, whereas before it contained a minimal
frame. Handle this by defaulting the operator to 'AND', which allows the
site to load and to be saved as is.

lib/nexpose/filter.rb

@@ -329,7 +329,9 @@ def <<(criterion)
     end
 
     def self.parse(json)
-      ret = Criteria.new([], json['operator'])
+      # The call returns empty JSON, so default to 'AND' if not present.
+      operator = json['operator'] || 'AND'
+      ret = Criteria.new([], operator)
       json['criteria'].each do |c|
         ret.criteria << Criterion.parse(c)
       end