Set empty ENV_CONVERSIONS record by default (#14566)

# Description

With Windows Path case-insensitivity in place, we no longer need an
`ENV_CONVERSIONS` for `PATH`, as the
`nu_engine::env::convert_env_values()` handles it automatically.

This PR:

* Removes the default `ENV_CONVERSIONS` for path from `default_env.nu`
* Sets `ENV_CONVERSIONS` to an empty record (so it can be `merge`'d) in
`main()` instead

# User-Facing Changes

No behavioral changes - Users will now have an empty `ENV_CONVERSIONS`
at startup by default, but the behavior should not change.

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

crates/nu-utils/src/default_files/default_env.nu

@@ -39,10 +39,3 @@ $env.PROMPT_COMMAND_RIGHT = $env.PROMPT_COMMAND_RIGHT? | default {||
 
     ([$last_exit_code, (char space), $time_segment] | str join)
 }
-
-$env.ENV_CONVERSIONS = {
-    "PATH": {
-        from_string: { |s| $s | split row (char esep) | path expand --no-symlink }
-        to_string: { |v| $v | path expand --no-symlink | str join (char esep) }
-    }
-}