fix declaration of object_uri DB field

Signed-off-by: Julien Veyssier <eneiluj@posteo.net>

lib/Migration/Version000013Date20190723185417.php

@@ -44,7 +44,7 @@ class Version000013Date20190723185417 extends SimpleMigrationStep {
 			$table->dropColumn('contact_uid');
 			$table->addColumn('object_uri', 'string', [
 				'notnull' => true,
-				'default' => '',
+				'default' => '--',
 				'length' => 64,
 			]);
 			$table->addIndex(['object_uri'], 'maps_object_uri');

lib/Migration/Version000102Date20190901152326.php

@@ -43,7 +43,7 @@ class Version000102Date20190901152326 extends SimpleMigrationStep {
 			$table = $schema->getTable('maps_address_geo');
 			$table->changeColumn('object_uri', [
 				'notnull' => true,
-				'default' => '',
+				'default' => '--',
 				'length' => 255,
 			]);
 		}