Fix some PHP 7.4 array index access issues

Summary: Ref T13518. See <https://discourse.phabricator-community.org/t/more-exceptions-when-viewing-diffs/3789/>. Under PHP 7.4, accessing an array index of values like `false` and `null` is no longer valid. This is great, but we occasionally do it.

Test Plan:
  - Upgraded to PHP 7.4.
  - Loaded revisions with added/changed lines, inlines, and Asana support configured.
  - Before patch: saw various fatals around accessing indexes of booleans and nulls.
  - After patch: clean revision.

Maniphest Tasks: T13518

Differential Revision: https://secure.phabricator.com/D21172

src/applications/differential/customfield/DifferentialAsanaRepresentationField.php

@@ -44,6 +44,10 @@ final class DifferentialAsanaRepresentationField
 
     $edge = head($edges[$src_phid][$edge_type]);
 
+    if (!$edge) {
+      return null;
+    }
+
     if (!empty($edge['data']['gone'])) {
       return phutil_tag(
         'em',

src/applications/differential/parser/DifferentialChangesetParser.php

@@ -1326,9 +1326,15 @@ final class DifferentialChangesetParser extends Phobject {
     $old_back = array();
     $new_back = array();
     foreach ($this->old as $ii => $old) {
+      if ($old === null) {
+        continue;
+      }
       $old_back[$old['line']] = $old['line'];
     }
     foreach ($this->new as $ii => $new) {
+      if ($new === null) {
+        continue;
+      }
       $new_back[$new['line']] = $new['line'];
     }
 

src/applications/differential/render/DifferentialChangesetTwoUpRenderer.php

@@ -593,9 +593,14 @@ final class DifferentialChangesetTwoUpRenderer
 
       $map = array();
       foreach ($new as $offset => $new_line) {
+        if ($new_line === null) {
+          continue;
+        }
+
         if ($new_line['line'] === null) {
           continue;
         }
+
         $map[$new_line['line']] = $offset;
       }