Return an empty list when no std help --find results are found (#13160)

# Description

Fixes #13143 by returning an empty list when there are no results found
by `std help --find/-f`

# User-Facing Changes

In addition, prints a message to stderr.

# Tests + Formatting

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

# After Submitting
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documentation](https://github.com/nushell/nushell.github.io) after the
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crates/nu-std/std/help.nu

@@ -772,6 +772,11 @@ You can also learn more at (ansi default_italic)(ansi light_cyan_underline)https
     let modules = (try { modules $target_item --find $find })
     if not ($modules | is-empty) { return $modules }
     
+    if ($find | is-not-empty) {
+        print -e $"No help results found mentioning: ($find)"
+        return []
+    }
+
     let span = (metadata $item | get span)
     error make {
         msg: ("std::help::item_not_found" | error-fmt)