Add new migrations to dav app

New migrations have been added to the dav app to
acheive the following:
1. Delete the existing entries in properties table
   which have null fileid entries
2. Restrict null value entry for fileid in properties table.

Signed-off-by: Sujith H <sharidasan@owncloud.com>

apps/dav/appinfo/Migrations/Version20190823065724.php

@@ -0,0 +1,62 @@
+<?php
+/**
+ * @author Sujith Haridasan <sharidasan@owncloud.com>
+ *
+ * @copyright Copyright (c) 2019, ownCloud GmbH
+ * @license AGPL-3.0
+ *
+ * This code is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License, version 3,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License, version 3,
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>
+ *
+ */
+
+namespace OCA\dav\Migrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use OCP\IDBConnection;
+use OCP\Migration\ISchemaMigration;
+
+/**
+ * Add NULL constraint to fileid column for properties table.
+ * The fileid column should not accept null values in the properties table.
+ */
+class Version20190823065724 implements ISchemaMigration {
+	/** @var IDBConnection  */
+	private $dbConnection;
+
+	public function __construct(IDBConnection $dbConnection) {
+		$this->dbConnection = $dbConnection;
+	}
+
+	public function changeSchema(Schema $schema, array $options) {
+		$prefix = $options['tablePrefix'];
+
+		$table = $schema->getTable("${prefix}properties");
+		$column = $table->getColumn('fileid');
+		/**
+		 * If the fileid column's notnull is set to false then
+		 * make sure before altering the column, that no entry
+		 * in the table has fileid with null. Else the migration
+		 * would fail. The below check does the same.
+		 */
+		if ($column->getNotnull() !== true) {
+			$qb = $this->dbConnection->getQueryBuilder();
+			$qb->delete('properties')
+				->where($qb->expr()->isNull('fileid'));
+			$qb->execute();
+
+			$table->changeColumn('fileid', [
+				'notnull' => true,
+			]);
+		}
+	}
+}

apps/dav/appinfo/info.xml

@@ -5,7 +5,7 @@
 	<description>ownCloud WebDAV endpoint</description>
 	<licence>AGPL</licence>
 	<author>owncloud.org</author>
-	<version>0.4.0</version>
+	<version>0.5.0</version>
 	<default_enable/>
 	<use-migrations>true</use-migrations>
 	<types>