Support "date" custom fields in "*.edit" endpoints

Summary: Fixes T13355. This didn't appear to be a ton of extra work, we just didn't get it for free in the original implementation in D14635.

Test Plan:
  - Saw "date" custom fields appear in Conduit API documentation for "maniphest.edit".
  - Set custom "date" field to null and non-null values via the API.

{F6666582}

Maniphest Tasks: T13355

Differential Revision: https://secure.phabricator.com/D20690

src/applications/conduit/parametertype/ConduitEpochParameterType.php

@@ -3,8 +3,24 @@
 final class ConduitEpochParameterType
   extends ConduitParameterType {
 
+  private $allowNull;
+
+  public function setAllowNull($allow_null) {
+    $this->allowNull = $allow_null;
+    return $this;
+  }
+
+  public function getAllowNull() {
+    return $this->allowNull;
+  }
+
   protected function getParameterValue(array $request, $key, $strict) {
     $value = parent::getParameterValue($request, $key, $strict);
+
+    if ($this->allowNull && ($value === null)) {
+      return $value;
+    }
+
     $value = $this->parseIntValue($request, $key, $value, $strict);
 
     if ($value <= 0) {

src/applications/transactions/editfield/PhabricatorEpochEditField.php

@@ -37,7 +37,8 @@ final class PhabricatorEpochEditField
   }
 
   protected function newConduitParameterType() {
-    return new ConduitEpochParameterType();
+    return id(new ConduitEpochParameterType())
+      ->setAllowNull($this->getAllowNull());
   }
 
 }

src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDate.php

@@ -226,20 +226,14 @@ final class PhabricatorStandardCustomFieldDate
     }
   }
 
-
-  public function shouldAppearInConduitTransactions() {
-    // TODO: Dates are complicated and we don't yet support handling them from
-    // Conduit.
-    return false;
-  }
-
   protected function newConduitSearchParameterType() {
     // TODO: Build a new "pair<epoch|null, epoch|null>" type or similar.
     return null;
   }
 
   protected function newConduitEditParameterType() {
-    return new ConduitEpochParameterType();
+    return id(new ConduitEpochParameterType())
+      ->setAllowNull(!$this->getRequired());
   }
 
   protected function newExportFieldType() {