allow 'note' to be null

Signed-off-by: Bjoern Schiessle <bjoern@schiessle.org>

core/Migrations/Version14000Date20180712153140.php

@@ -21,6 +21,7 @@
 
 namespace OC\Core\Migrations;
 
+use OCP\DB\ISchemaWrapper;
 use OCP\Migration\SimpleMigrationStep;
 
 /**
@@ -35,10 +36,7 @@ class Version14000Date20180712153140 extends SimpleMigrationStep {
 		$schema = $schemaClosure();
 
 		$table = $schema->getTable('share');
-		$table->addColumn('note', 'text', [
-			'notnull' => true,
-			'default' => ''
-		]);
+		$table->addColumn('note', 'text');
 
 		return $schema;
 	}

lib/private/Share20/Share.php

@@ -322,8 +322,11 @@ class Share implements \OCP\Share\IShare {
 	 * @inheritdoc
 	 */
 	public function getNote() {
+		if (is_string($this->note)) {
 			return $this->note;
 		}
+		return '';
+	}
 
 	/**
 	 * @inheritdoc