fix(vim.system): env=nil passes env=nil to uv.spawn

731e616a79 made it so passing `{env = nil, clear_env = true }` would
pass `{env = {}}` to `vim.uv.spawn`.

However this is not what `clear_env` is (arguably) supposed to do.
If `env=nil` then that implies the uses wants `vim.uv.spawn()` to use
the default environment. Adding `clear_env = true` simply prevents
`NVIM` (the base environment) from being added.

Fixes #34730

(cherry picked from commit 4eebc46930)

runtime/lua/vim/_editor.lua

@@ -106,7 +106,7 @@ local utfs = {
 ---   - env: table<string,string> Set environment variables for the new process. Inherits the
 ---     current environment with `NVIM` set to |v:servername|.
 ---   - clear_env: (boolean) `env` defines the job environment exactly, instead of merging current
----     environment.
+---     environment. Note: if `env` is `nil`, the current environment is used but without `NVIM` set.
 ---   - stdin: (string|string[]|boolean) If `true`, then a pipe to stdin is opened and can be written
 ---     to via the `write()` method to SystemObj. If string or string[] then will be written to stdin
 ---     and closed. Defaults to `false`.