Fix _discard leaking into SQL for multi-select enum filters

[tonekk]

Summary

When an enum filter is used in multi-select mode, _discard (the "..." placeholder) gets submitted as a literal value on subsequent page loads, producing SQL like:

WHERE (users.role IN ('_discard','admin','editor'))

This PR fixes both the client-side and server-side causes.

Steps to reproduce

1. Have a model with an enum field (e.g. role on User)
2. Navigate to the index page and open the filter for that enum field
3. Select multiple values (e.g. admin and editor) and submit the filter
4. On the filtered results page, do not touch the filter select — just click "Filter" (submit) again
5. Observe the generated SQL: _discard appears as a value in the IN (...) clause

Root cause

Client-side (src/rails_admin/filter-box.js): When restoring a multi-select filter on page reload, _discard remains implicitly selected because .prop("multiple", true) preserves the default selection, and the code only hides the option without deselecting it.

Server-side (lib/rails_admin/abstract_model.rb): The _discard guard only checks scalar equality (v == '_discard'), which is always false when @value is an array like ['_discard', 'admin', 'editor'].

Fix

• Client-side: Add .prop("selected", false) when hiding special options in multi-select mode
• Server-side: Strip _discard from array values as defense-in-depth; return nil if the array becomes empty after stripping