[FIX] web: not loose footer in some case 08b2d1f9a

08b2d1f9ac was intended so margin on the top of the body worked (it
seems that wkhtmtopdf only started the body from the first visible
elements).

From a subsequent report, on some document template the CSS could have
adverse effect of not showing the footer.

With this changeset, the CSS is only applied on the body which was the
first intention.

fixes #31531
opw-1925894
closes #31533

addons/web/static/src/scss/report.scss

@@ -36,7 +36,7 @@ div.address.row {
 }
 
 /* margin-top does not work on first visible element of page */
-body > *:first-child::before {
+body:not([onload="subst()"]) > *:first-child::before {
     content: '\00a0';
     display: block;
     height: 1px;