kpt not preserving comment only objects

[nduja]

Expected behavior

kpt fn eval preserves all comments in a yam file.

Actual behavior

kpt fn eval removes objects/documents made up of only comments.

Information

kpt version: 1.0.0-beta.31

In our packages we use these "comment only" objects for headers/context for humans and would like to preserve them

Steps to reproduce the behavior

1. Create a yaml file with an object of entirely comments.

   # Comment only object
   ---
   # Comment in object
   apiVersion: v1
   kind: Namespace
   metadata:
     name: test-ns
   

2. Run a kpt fn eval function
   kpt fn eval --image gcr.io/kpt-fn/search-replace:v0.2.0 --exclude-kind 'Namespace' -- 'by-path=metadata.name' "put-value=test-ns"
3. Inspect file, "# Comment only object" is removed.

   $ cat ns.yaml 
   # Comment in object
   apiVersion: v1
   kind: Namespace
   metadata:
     name: test-ns
   

[yuwenma]

I think the problem is not dedicated to a specific function, but how kpt reads/writes and preserves the yaml content during passing them to the KRM functions. @droot do you mind taking a look at this issue?