Fix JAVA_HOME/JDK_HOME handling in set-java-home.nu for Nushell

[HaaaaE]

Nushell has changed a lot since set-java-home.nu was last updated.

This PR fixes errors and makes the script more robust.

Changes in set-java-home.nu and reasons:

Use an env-aware function definition

• Change: def → def --env
• Reason: environment-variable updates should be performed inside an --env function so they reliably affect the caller environment.

Handle command execution results explicitly

• Change: replace direct let java_path = (asdf which java) with:
  let result = (do -i { asdf which java } | complete) and a match on { exit_code, stdout }
• Reason: makes success/failure paths explicit, improves readability, and avoids relying on implicit truthiness of command output.

Make the “no Java available” path safe and deterministic

• Change: when asdf which java fails or returns empty/whitespace output, clear the variables:
  hide-env -i JAVA_HOME JDK_HOME
• Reason: prevents stale JAVA_HOME/JDK_HOME values from persisting when the Java tool is not available in the current context.

Remove dependency on realpath and the deprecated nth

• Change: replace realpath, nth 0 with path expand, first
• Reason: use Nushell built-in command path expand to replace realpath is recommended by Nushell community. And nth is not available in the newest Nushell version.

Automatically refresh on prompt

• Change: append asdf_update_java_home to $env.config.hooks.pre_prompt
• Reason: keeps JAVA_HOME/JDK_HOME in sync during interactive use without requiring manual invocation.

No behavior change is intended when asdf which java returns a valid path.

This change primarily improves robustness in error and empty-output cases and aligns the script with current Nushell environment semantics.

Thanks.