handle NONE type result from rapidyaml parser

Fixes #1270

Author: johnbartholomew

core/vm.cpp

@@ -1703,6 +1703,9 @@ class Interpreter {
                     j.push_back(yamlTreeToJson(doc));
                 }
             }
+        } else if (tree.type(tree.root_id()).is_notype()) {
+            scratch = makeNull();
+            return nullptr;
         } else {
             j = yamlTreeToJson(tree);
         }

test_suite/stdlib.jsonnet

@@ -1478,6 +1478,9 @@ std.assertEqual(std.parseJson('12'), 12) &&
 std.assertEqual(std.parseJson('12.123'), 12.123) &&
 std.assertEqual(std.parseJson('{"a": {"b": ["c", 42]}}'), { a: { b: ['c', 42] } }) &&
 
+// Empty input evaluates to `null`.
+// Regression test for https://github.com/google/jsonnet/issues/1270
+std.assertEqual(std.parseYaml('   \n'), null) &&
 std.assertEqual(std.parseYaml('{}'), {}) &&
 std.assertEqual(std.parseYaml('[]'), []) &&
 std.assertEqual(