Bug #1514641: Avoid undefined variable 'css' in block render

To avoid lines in error log

Behatnotneeded

Change-Id: I7024d0c1aa764d27c7d3a03dc8c01ae2e6031562
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
(cherry picked from commit f600592d)

htdocs/blocktype/lib.php

@@ -899,6 +899,7 @@ class BlockInstance {
                 log_debug($e->getMessage());
                 $content = '';
                 $js = '';
+                $css = '';
             }
 
             if (!defined('JSON') && !$jsreply) {