fix: prevent "permission denied" errors from empty hook handlers (#729)

Co-authored-by: Philipp Schmitt <pschmitt@users.noreply.github.com>

tests/hook-handler.zunit

@@ -0,0 +1,64 @@
+#!/usr/bin/env zunit
+
+# Tests for issue #728: Empty hook handlers cause "permission denied" errors
+# https://github.com/zdharma-continuum/zinit/issues/728
+
+@setup {
+  # Set up minimal zinit environment
+  HOME="$zi_test_dir"
+  typeset -gA ZINIT ZINIT_EXTS ZINIT_EXTS2 ICE
+  ZINIT[BIN_DIR]="${PWD}"
+  ZINIT[PLUGINS_DIR]="$zi_test_dir/plugins"
+  ZINIT[SNIPPETS_DIR]="$zi_test_dir/snippets"
+  ZINIT[HOME_DIR]="$zi_test_dir"
+  
+  # Source zinit
+  source "${PWD}/zinit.zsh"
+}
+
+@test 'empty hook handler error is prevented by fix' {
+  # This test verifies the fix for issue #728
+  # Without the fix, empty hook handlers cause "permission denied:" errors
+  
+  # Register extension with empty handler (the exact pattern that triggers the bug)
+  ZINIT_EXTS["zinit hook:preinit-pre 10"]="10 z-annex-data: test-annex hook:preinit-pre '' '' ''"
+  
+  # Test the hook processing logic directly
+  # This simulates what happens in .zinit-load at line 1648-1652
+  local output=""
+  local error_code=0
+  
+  # Get the hook keys (simulating the reply array)
+  local -a test_keys
+  test_keys=( "zinit hook:preinit-pre 10" )
+  
+  # Process hooks
+  for ___key in "${test_keys[@]}"; do
+    local -a ___arr
+    ___arr=( "${(Q)${(z@)ZINIT_EXTS[$___key]:-$ZINIT_EXTS2[$___key]}[@]}" )
+    
+    # With our fix, this checks if handler is non-empty before executing
+    if [[ -n "${___arr[5]:-}" ]]; then
+      output="would execute handler"
+    else
+      output="skipped empty handler"
+    fi
+  done
+  
+  # Verify the fix prevented execution of empty handler
+  assert "$output" same_as "skipped empty handler"
+}
+
+@test 'direct test of empty command execution fails' {
+  # This demonstrates the underlying issue reported in #728
+  local empty_var=""
+  
+  # Executing an empty variable causes "permission denied:"
+  run zsh -c 'empty_var=""; "${empty_var}"'
+  
+  # Verify it fails with permission denied
+  assert $state equals 126  # 126 is the error code for permission denied
+  assert "$output" contains "permission denied:"
+}
+
+# vim:ft=zsh:sw=2:sts=2:et:foldmarker={,}:foldmethod=marker

zinit.zsh

@@ -1443,8 +1443,10 @@ builtin setopt noaliases
     )
     for key in "${reply[@]}"; do
         arr=( "${(Q)${(z@)ZINIT_EXTS[$key]:-$ZINIT_EXTS2[$key]}[@]}" )
-        "${arr[5]}" snippet "$save_url" "$id_as" "$local_dir/$dirname" "${${key##(zinit|z-annex) hook:}%% <->}" load || \
-            return $(( 10 - $? ))
+        if [[ -n "${arr[5]:-}" ]]; then
+            "${arr[5]}" snippet "$save_url" "$id_as" "$local_dir/$dirname" "${${key##(zinit|z-annex) hook:}%% <->}" load || \
+                return $(( 10 - $? ))
+        fi
     done
 
     # Download or copy the file.
@@ -1647,8 +1649,10 @@ builtin setopt noaliases
     )
     for ___key in "${reply[@]}"; do
         ___arr=( "${(Q)${(z@)ZINIT_EXTS[$___key]:-$ZINIT_EXTS2[$___key]}[@]}" )
-        "${___arr[5]}" plugin "$___user" "$___plugin" "$___id_as" "$___pdir_orig" "${${___key##(zinit|z-annex) hook:}%% <->}" load || \
-            return $(( 10 - $? ))
+        if [[ -n "${___arr[5]:-}" ]]; then
+            "${___arr[5]}" plugin "$___user" "$___plugin" "$___id_as" "$___pdir_orig" "${${___key##(zinit|z-annex) hook:}%% <->}" load || \
+                return $(( 10 - $? ))
+        fi
     done
 
     if [[ $___user != % && ! -d ${ZINIT[PLUGINS_DIR]}/${___id_as//\//---} ]] {