[Regression] Passing undefined to select.add should fallback to the default value

Regression caused by r170259.

Passing an undefined as the second argument of select.add should be handled by falling back to the default value of the optional argument.

TEST=Automated. Added to select-add.html
BUG=363014

Review URL: https://codereview.chromium.org/238613002

git-svn-id: svn://svn.chromium.org/blink/trunk@171747 bbb929c8-8fbe-4397-9dbb-9b2b20218538