fix: prevent "permission denied" errors from empty hook handlers

[nicksherron]

Fixes #728

Summary

This PR fixes the "permission denied: " errors that occur when zinit tries to execute empty hook handlers from annexes.

Problem

When loading certain annexes, zinit attempts to execute hook handlers that may be empty or undefined. This results in the shell trying to execute an empty command, which produces "permission denied: " errors:

.zinit-load:25: permission denied:
.zinit-load-snippet:63: permission denied:

Solution

Added checks to ensure hook handlers are non-empty before attempting to execute them:

1. In .zinit-load() function: Check if ${___arr[5]} is non-empty before execution
2. In .zinit-load-snippet() function: Check if ${arr[5]} is non-empty before execution

Changes

• Modified zinit.zsh:
  • Added conditional check in .zinit-load() at line 1650
  • Added conditional check in .zinit-load-snippet() at line 1446
• Added comprehensive tests in tests/empty-hook-handler.zunit to ensure:
  • Empty handlers don't cause errors
  • Valid handlers still execute normally
  • Mixed scenarios work correctly

Testing

1. Manual testing confirmed the fix resolves the issue
2. Added automated tests that verify:
   • Empty hook handlers in ZINIT_EXTS don't cause errors
   • Empty hook handlers in ZINIT_EXTS2 don't cause errors
   • Empty snippet hook handlers don't cause errors
   • Valid hook handlers continue to work as expected
   • Mixed scenarios with both empty and valid handlers work correctly

Backwards Compatibility

This change is fully backwards compatible. It only adds a safety check and doesn't change any existing behavior for valid hook handlers.

Related Issues

This issue has been reported by multiple users experiencing "permission denied" errors when using zinit annexes.