[vim/vim] serverlist() fails when there is no connection to the X server (PR #21124)

## Problem

Since patch 9.2.0818 (#20716), `serverlist()` gives `E240: No connection to the X server` when the X11 clientserver backend cannot connect to the X server — for example on a Wayland-only system or with `$DISPLAY` unset. Before that patch it returned an empty result.

## Why this is a regression

- **It contradicts the documentation.** `:help serverlist()` says *"When there are no servers or the information is not available an empty string is returned"*, and 9.2.0818 did not update it.
- **The error was deliberately removed before.** Patch 6.0.191: *"When no servers are available serverlist() gives an error instead of returning an empty string"* → *"Don't give an error message"*. Patch 6.1.349 did the same for `--serverlist` when the display cannot be opened.
- **Entry points and backends now disagree.** In exactly the situation where the function fails, the `--serverlist` command line argument still prints an empty list, and the socket backend still returns an empty list.
- **The return type is broken on the failing path.** `rettv` is never set, so the result is a Number rather than a String or a List:
  ```vim
  silent! let r = serverlist()
  echo type(r)   " 0 (Number), r == 0
  ```

## Solution

Do not give an error when the X server cannot be reached; return an empty string, or an empty `List` when the `"list"` option is used.

The `"list"` option now always results in a `List` when no server names are available. That is the type inconsistency 9.2.0818 set out to fix — `serverlist({'list': 1})` returning a String — and it is fixed for every backend, including a build without `+clientserver`.

## Tests

`Test_clientserver_serverlist_without_x11()` now checks that a child Vim without `$DISPLAY` returns `''` from `serverlist()` and `[]` from `serverlist(#{list: v:true})`. It fails before the fix and passes after it:

```
# before
Found errors in Test_clientserver_serverlist_without_x11():
... line 36: Expected 0 but got 1
# after
Executed 1 test
```

The child process wraps the calls in a try/catch because an error in a sourced script only aborts the failing command: without it the assertion is never reached, `v:errors` stays empty and the test passes even when `serverlist()` fails (verified: exit code 0).

The `catch /E240:/` workaround that 9.2.0818 added to `Test_remote_serverlist()` in `test_vim9_builtin.vim` is no longer needed and is removed, so that test checks the return types again.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/21124

-- Commit Summary --

  * serverlist() fails when there is no connection to the X server

-- File Changes --

    M runtime/doc/builtin.txt (3)
    M src/clientserver.c (17)
    M src/testdir/test_clientserver.vim (18)
    M src/testdir/test_vim9_builtin.vim (12)

-- Patch Links --

https://github.com/vim/vim/pull/21124.patch
https://github.com/vim/vim/pull/21124.diff

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