[FIX] web: stat button string in a span

With only the module appointment installed, go to Calendar App,
Online Appointments and click on "Create". In the form view displayed a
stat button "Customer Preview" is visible but is badly styled. This is
due to the fact that "Customer Preview" is no longer wrap by a span since
the rewriting of the form view (see odoo/odoo@48ef812a63).
We fix the problem by making the default slot content of ViewButton be a
span wrapping the prop string (if any).

Part-of: odoo/odoo#98072

addons/web/static/src/views/view_button/view_button.xml

@@ -18,7 +18,7 @@
         t-on-click.stop="onClick"
     >
       <t t-if="icon" t-tag="icon.tag" t-att-class="icon.class" t-att-src="icon.src"/>
-      <t t-slot="contents"><t t-esc="props.string"/></t>
+      <t t-slot="contents"><span t-if="props.string" t-esc="props.string"/></t>
     </t>
   </t>