Allow NULL in expressions.

Author: udan11

src/Components/Expression.php

@@ -186,6 +186,7 @@ public static function parse(Parser $parser, TokensList $list, array $options =
             if (($token->type === Token::TYPE_KEYWORD)
                 && ($token->flags & Token::FLAG_KEYWORD_RESERVED)
                 && ($token->value !== 'DUAL')
+                && ($token->value !== 'NULL')
             ) {
                 // Keywords may be found only between brackets.
                 if ($brackets === 0) {

src/Components/ExpressionArray.php

@@ -72,6 +72,7 @@ public static function parse(Parser $parser, TokensList $list, array $options =
                 && ($token->flags & Token::FLAG_KEYWORD_RESERVED)
                 && ((~$token->flags & Token::FLAG_KEYWORD_FUNCTION))
                 && ($token->value !== 'DUAL')
+                && ($token->value !== 'NULL')
             ) {
                 // No keyword is expected.
                 break;

src/Statement.php

@@ -170,7 +170,6 @@ public function build()
                 if (!empty($built[$field])) {
                     continue;
                 }
-
                 $built[$field] = true;
             }