[vim/vim] Terminal: blank lines don't use the background color specified with hl-terminal in the terminal window (anymore) (Issue #21087)

bfrg created an issue (vim/vim#21087)

### Steps to reproduce

Run `gvim --clean -S test.vim` with the following `test.vim`:
```vim
vim9script

highlight Terminal guibg=darkred

const text: list<string> =<< trim EOF
    test

    test
    test


    test
EOF

term_start(['sh', '-c', $'printf "{join(text, "\n")}"'])
```

**Result:** Blank lines are not highlighted using the background color specified with the `hl-terminal` highlight group.

**Screenshot:**
<img width="930" height="737" alt="Image" src="https://github.com/user-attachments/assets/5725d3a9-c61e-4831-8a4a-01b27a744760" />

### Expected behaviour

Since the background of the terminal window is specified to use `darkred`, I'm expecting _all_ lines (blanks and non-blanks) to use the same background color. This used to work in the past.

This happens in both Gvim and terminal Vim.

### Version of Vim

9.2.910

### Environment

- Linux
- Gvim
- Vim, any terminal.

### Logs and stack traces

```shell

```

-- 
Reply to this email directly or view it on GitHub:
https://github.com/vim/vim/issues/21087
You are receiving this because you are subscribed to this thread.

Message ID: <vim/vim/issues/[email protected]>

-- 
-- 
You received this message from the "vim_dev" maillist.
Do not top-post! Type your reply below the text you are replying to.
For more information, visit http://www.vim.org/maillist.php

--- 
You received this message because you are subscribed to the Google Groups "vim_dev" group.
To unsubscribe from this group and stop receiving emails from it, send an email to [email protected].
To view this discussion visit https://groups.google.com/d/msgid/vim_dev/vim/vim/issues/21087%40github.com.