I'm trying to modify issue.search.html to allow one to search for an 
empty nosy list. I defined a macro search_select_multi in page.html that 
is identical to the search_select macro except that it allows for 
multiple selections:

<!-- like search_select, but allows multiple selections. -->
<td metal:define-macro="search_select_multi">
  <select tal:attributes="name name; id name; multiple string:true; size 
string:2"
          tal:define="value python:request.form.getvalue(name)">
    <option selected value="" i18n:translate="">don't care</option>
    <metal:slot define-slot="extra_options" />
    <option value="" i18n:translate="" disabled="disabled">------------
</option>
    <option tal:repeat="s python:db[db_klass].list()"
            tal:attributes="value s/id; selected python:value == s.id"
            tal:content="python:s[db_content]"></option>
  </select>
</td>

Here is the reference to the search_select_multi macro:

<tr tal:define="name string:nosy;
                db_klass string:user;
                db_content string:address;"
    tal:condition="db/user/is_view_ok">
  <th i18n:translate="">Email List:</th>
  <td metal:use-macro="search_select_multi">
  </td>
  <td metal:use-macro="column_input"></td>
  <td>&nbsp;</td>
  <td>&nbsp;</td>
</tr>

I would have thought that this line in the search_select_multi macro:

 <option selected value="" i18n:translate="">don't care</option>

might allow one to search for an empty nosy list, When the <option> 
value of "" is used it matches all nosy lists, even empty ones. What 
<option> value does one specify when one wants to search for issues that 
have an empty nosy list?