Use IDL argument default values in modules/encoding/

Replace bogus 'null' default values for DOMString/ScalarValueString
arguments with the actual default values from the spec, and remove the
handling of null strings in the C++ implementation.

BUG=258153

Review URL: https://codereview.chromium.org/356703002

git-svn-id: svn://svn.chromium.org/blink/trunk@176934 bbb929c8-8fbe-4397-9dbb-9b2b20218538