Handle empty maps when converting YAML to a string.

sdk/lib/_internal/pub/test/utils_test.dart

Index: sdk/lib/_internal/pub/test/utils_test.dart
diff --git a/sdk/lib/_internal/pub/test/utils_test.dart b/sdk/lib/_internal/pub/test/utils_test.dart
index c1c29a811c73e894f7ef9fa3ca36525bfdf93036..a137d2d079094cfa0ad8429dec33f37955513b1e 100644
--- a/sdk/lib/_internal/pub/test/utils_test.dart
+++ b/sdk/lib/_internal/pub/test/utils_test.dart
@@ -80,5 +80,12 @@ no: 1
 null: null
 true: bool"""));
     });
+
+    test('handles empty maps', () {
+      expect(yamlToString({}), equals("{}"));
+      expect(yamlToString({'a': {}, 'b': {}}), equals("""
+a: {}
+b: {}"""));
+    });
   });
 }