prevent convert exception, if image couldn't loaded

Application/views/tpl/documents/inc/header.tpl

@@ -3,7 +3,10 @@
 [{block name="pdfToping"}]
     <div class="pdf_header_positioning">
         [{if $showLogo}]
-            <img alt="" src="[{$oViewConf->getImageUrl('Elektroversand-Schmidt_Logo_180.jpg')}]">
+            [{assign var="logoUrl" value=$oViewConf->getImageUrl('Elektroversand-Schmidt_Logo_180.jpg')}]
+            [{if $logoUrl}]
+                <img alt="" src="[{$logoUrl}]">
+            [{/if}]
         [{/if}]
     </div>
 [{/block}]