For people who may struggle fixing the source code itself, there's a much simpler, but equally effective solution. If the member has not made any posts, then the variable $you gets passed as a SQL query. Hence inside the template you can add this little workaround:
{{if !(is_numeric($you))}}
<span>0</span>
{{else}}
<span>{expression="\IPS\Member::loggedIn()->language()->formatNumber( $you )"}</span>
{{endif}}
To edit the template go to: AdminCP -> Customization -> Edit HTML and CSS -> Templates -> jimmotoppostersplus -> jimmoTopPostersPlusRows
EDIT: This only replaces the 1 with a 0 if you have no posts that week.