Handle missing references

js/test_cases/missing_reference/input1.json

@@ -0,0 +1 @@
+{"rank": "Admiral"}

js/test_cases/missing_reference/main.dust

@@ -0,0 +1 @@
+Hello {name}!

src/renderer/renderer.rs

@@ -87,8 +87,13 @@ impl<'a> DustRenderer<'a> {
         match tag {
             DustTag::DTComment(_comment) => (),
             DustTag::DTReference(reference) => {
-                let val = walk_path(context, &reference.path.keys)?;
-                return val.render();
+                let val = walk_path(context, &reference.path.keys);
+                if let Err(RenderError::WontWalk { .. }) = val {
+                    // If reference does not exist in the context, it becomes an empty string
+                    return Ok("".to_owned());
+                } else {
+                    return val?.render();
+                }
             }
             _ => (), // TODO: Implement the rest
         }