yaml_preprocessor: fix processing empty sequences to null

* Fix bug where preprocessing an empty list converts it to null

flatland_server/src/yaml_preprocessor.cpp

@@ -77,7 +77,7 @@ void YamlPreprocessor::ProcessNodes(YAML::Node &node,
       // copy the elements to a new sequence, checking for $[include] expressions
       // as we go. If an include is found, replace it with one or more nodes
       // parsed from the included file.
-      YAML::Node new_sequence;
+      YAML::Node new_sequence = YAML::Node(YAML::NodeType::Sequence);
       for (YAML::Node child : node) {
         std::vector<YAML::Node> included_nodes = {};
         if (ProcessSequenceIncludeNode(included_nodes, child, ref_path)) {

flatland_server/test/yaml_preprocessor/yaml/include.parent.out.yaml

@@ -12,3 +12,4 @@ f:
   - second element
   - third element
   - parent last element
+g: []

flatland_server/test/yaml_preprocessor/yaml/include.parent.yaml

@@ -7,3 +7,4 @@ f:
   - parent first element
   - $[include] include.sequence.yaml
   - parent last element
+g: [] # Test: make sure empty lists are not modified

flatland_server/test/yaml_preprocessor/yaml_preprocessor_test.cpp

@@ -158,6 +158,10 @@ TEST(YamlPreprocTest, testIncludeStrings) {
   compareNodes("f", 2, in, out);
   compareNodes("f", 3, in, out);
   compareNodes("f", 4, in, out);
+
+  // previously we had a bug where empty lists would be converted to null
+  EXPECT_EQ(YAML::NodeType::Sequence, in["g"].Type());
+  EXPECT_EQ(0, in["g"].size());
 }
 
 // Run all the tests that were declared with TEST()