Likely, fix a warning when rendering modified coverage

Summary: See PHI1819. This structure may have `null` elements.

Test Plan: Will confirm user reproduction case.

Differential Revision: https://secure.phabricator.com/D21420

src/applications/differential/parser/DifferentialChangesetParser.php

@@ -1327,6 +1327,10 @@ final class DifferentialChangesetParser extends Phobject {
     $not_covered = 0;
 
     foreach ($this->new as $k => $new) {
+      if ($new === null) {
+        continue;
+      }
+
       if (!$new['line']) {
         continue;
       }