Fix null default values not being returned by defaultValues()

Refs #5454

src/Database/Schema/Table.php

@@ -342,7 +342,7 @@ class Table {
 	public function defaultValues() {
 		$defaults = [];
 		foreach ($this->_columns as $name => $data) {
-			if (isset($data['default'])) {
+			if (array_key_exists('default', $data)) {
 				$defaults[$name] = $data['default'];
 			}
 		}

tests/TestCase/Database/Schema/TableTest.php

@@ -164,11 +164,13 @@ class TableTest extends TestCase {
 			'default' => 'A title'
 		])->addColumn('body', [
 			'type' => 'text',
+			'default' => null,
 		]);
 		$result = $table->defaultValues();
 		$expected = [
 			'id' => 0,
-			'title' => 'A title'
+			'title' => 'A title',
+			'body' => null
 		];
 		$this->assertEquals($expected, $result);
 	}