[FIX] web_editor: prevent adding sandbox

in 28740b11da37239953185478de9f391265db543f
we add an option for a sanboxed iframe.

This mistakenly uses `null` instead of `false` in a `t-att`.
Which sets the sandbox attribute, breaking `mass_mail`.

We replace `null` with `false`, which correctly does not add the attribute
 at all.

 task-3255777

closes odoo/odoo#121869

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/web_editor/static/src/js/backend/html_field.xml

@@ -4,7 +4,7 @@
     <t t-name="web_editor.HtmlField" owl="1">
         <t t-if="props.readonly || props.notEditable || (sandboxedPreview and !state.showCodeView)">
             <t t-if="this.showIframe">
-                <iframe t-ref="iframe" t-att-class="{'d-none': !this.state.iframeVisible, 'o_readonly': true}" t-att-sandbox="sandboxedPreview ? 'allow-same-origin' : null"></iframe>
+                <iframe t-ref="iframe" t-att-class="{'d-none': !this.state.iframeVisible, 'o_readonly': true}" t-att-sandbox="sandboxedPreview ? 'allow-same-origin' : false"></iframe>
             </t>
             <t t-else="">
                 <div  t-ref="readonlyElement" class="o_readonly" t-out="markupValue" />