Can't create a new empty file on GitHub using API

create_github_files.sh

# This works
curl -u USER:PASS https://api.github.com/repos/USER/REPO/contents/a.txt --request PUT --data-ascii $'{"path": "a.txt", "content": "bXkgbmV3IGZpbGUgY29udGVudHM=", "message": "hello"}'

# Does not work
curl -u USER:PASS https://api.github.com/repos/USER/REPO/contents/a.txt --request PUT --data-ascii $'{"path": "a.txt", "content": "", "message": "hello"}'

# HTTP status 422. Body:
# {
#   "message": "content is not valid Base64",
#   "documentation_url": "http://developer.github.com/v3/repos/contents/"
# }

# However, this creates an empty (0 bytes) file:
echo "" | base64 --decode > tmp.txt && ls -al tmp.txt