MDL-45084 theme: File was not chunked on IE

This only applied when themedesignermode was disabled, because the
first chunk is 0, but 0 is resolved to false. We needed to compare
$chunk with null in order to know if it was set.

theme/styles.php

@@ -198,7 +198,7 @@
     // let's ignore legacy IE breakage here too.
     css_send_uncached_css($csscontent);
 
-} else if ($chunk and file_exists($candidatesheet)) {
+} else if ($chunk !== null and file_exists($candidatesheet)) {
     // Greetings stupid legacy IEs!
     css_send_cached_css($candidatesheet, $etag);