Automatically quoting strings passed to isNull and isNotNull

All other methods in QueryExpression had the same behavior so it
made sense to support that here too. Fixes #3836

src/Database/Expression/QueryExpression.php

@@ -202,7 +202,8 @@ public function lte($field, $value, $type = null) {
 /**
  * Adds a new condition to the expression object in the form "field IS NULL".
  *
- * @param string|ExpressionInterface $field database field to be tested for null
+ * @param string|\Cake\Database\ExpressionInteface $field database field to be
+ * tested for null
  * @return QueryExpression
  */
 	public function isNull($field) {
@@ -215,7 +216,8 @@ public function isNull($field) {
 /**
  * Adds a new condition to the expression object in the form "field IS NOT NULL".
  *
- * @param string $field database field to be tested for not null
+ * @param string|\Cake\Database\ExpressionInteface $field database field to be
+ * tested for not null
  * @return QueryExpression
  */
 	public function isNotNull($field) {

tests/TestCase/Database/QueryTest.php

@@ -2515,6 +2515,27 @@ public function testIsNullWithExpressions() {
 		$this->assertEmpty($result->fetchAll('assoc'));
 	}
 
+/**
+ * Tests that strings passed to isNull and isNotNull will be treaded as identifiers
+ * when using autoQuoting
+ *
+ * @return void
+ */
+	public function testIsNullAutoQuoting() {
+		$this->connection->driver()->autoQuoting(true);
+		$query = new Query($this->connection);
+		$query->select('*')->from('things')->where(function($exp) {
+			return $exp->isNull('field');
+		});
+		$this->assertQuotedQuery('WHERE \(<field>\) IS NULL', $query->sql());
+
+		$query = new Query($this->connection);
+		$query->select('*')->from('things')->where(function($exp) {
+			return $exp->isNotNull('field');
+		});
+		$this->assertQuotedQuery('WHERE \(<field>\) IS NOT NULL', $query->sql());
+	}
+
 /**
  * Assertion for comparing a table's contents with what is in it.
  *