Empty lines in indented heredoc ruins the experience

When an indented heredoc contains an empty line, modern editors strip off all trailing whitespace, which then leaves all the leading whitespaces in.

Example:

```hcl
item = <<-EOF
foo

bar
EOF
```

### Expected behavior

It should act like `item = "foo\n\nbar\n"`

### Actual behavior

It acts like `item = " foo\n\n bar\n"`

### References
no known references

Start by reproducing the indented heredoc example from the issue, including the empty line, and inspect the heredoc parsing path in the HCL repository. Done means the example evaluates like "foo\n\nbar\n" rather than retaining indentation after the empty line; add or update coverage for this case.

Written by the indexing model from the issue text.