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)
e732cbe36c