add explicit true type == "no filter"

include/llmr/map/filter_expression.hpp

@@ -48,7 +48,8 @@ class PropertyExpression;
 
 typedef util::variant<
     util::recursive_wrapper<PropertyFilter>,
-    util::recursive_wrapper<PropertyExpression>
+    util::recursive_wrapper<PropertyExpression>,
+    std::true_type
 > PropertyFilterExpression;
 
 

include/llmr/style/bucket_description.hpp

@@ -117,7 +117,7 @@ public:
     std::string source_name;
     std::string source_layer;
 
-    PropertyFilterExpression filter;
+    PropertyFilterExpression filter = std::true_type();
 
     // Specifies how the geometry for this bucket should be created
     BucketGeometryDescription geometry;