[FIX] account: show the 'reference' field in report

Some module, such as `l10n_be_invoice_bba`, use the field to provide
information to customers. Basically, if this field is filled in, we
should print it.

This reverts commit 59efea7e31.

opw-748930

addons/account/views/report_invoice.xml

@@ -45,7 +45,7 @@
                     <strong>Customer Code:</strong>
                     <p t-field="o.partner_id.ref"/>
                 </div>
-                <div name="reference" class="col-xs-2" t-if="o.reference and o.type == 'in_invoice'">
+                <div name="reference" class="col-xs-2" t-if="o.reference">
                     <strong>Reference:</strong>
                     <p t-field="o.reference"/>
                 </div>