Add a test for parsing the NULL literal

(Coveralls notices we didn't have one.)

tests/sqlparser_common.rs

@@ -401,6 +401,16 @@ fn parse_projection_nested_type() {
     //TODO: add assertions
 }
 
+#[test]
+fn parse_null_in_select() {
+    let sql = "SELECT NULL";
+    let select = verified_only_select(sql);
+    assert_eq!(
+        &ASTNode::SQLValue(Value::Null),
+        expr_from_projection(only(&select.projection)),
+    );
+}
+
 #[test]
 fn parse_escaped_single_quote_string_predicate() {
     use self::ASTNode::*;