Your code should work, but you’re using your helper
{{sample}}
in the template without passing it any arguments. Even if you’re testing to see if things work, pass it a test string like
{{sample "some string"}}
You need to do that because you’re using the helper in a raw hbr template. For testing purposes, the helper should work even if you don’t pass it anything in regular hbs templates.
If you’re creating a block-helper, then it needs to have # and you need to close it like so.
{{#sample}}
{{/sample}}
Adding ~ to your helper optionally trims whitespace between any HTML tags it returns. So
{{~#sample}}
{{/sample}}