Prevent a PHP crash when the icon tag is missing from a highlight_code definition in the XML.

git-svn-id: http://svn.code.sf.net/p/itop/code/trunk@3476 a333f486-631f-4898-b8df-5754b55c2be0

setup/compiler.class.inc.php

@@ -1092,6 +1092,10 @@ EOF;
 						$sIcon = $sModuleRelativeDir.'/'.$sIcon;
 						$sIcon = "utils::GetAbsoluteUrlModulesRoot().'$sIcon'";
 					}
+					else
+					{
+						$sIcon = "''";
+					}
 					switch($sColor)
 					{
 						// Known PHP constants: keep the literal value as-is