Drop useless String::isNull() check in V8CSSStyleDeclaration::namedPropertyGetterCustom()

Drop useless String::isNull() check in V8CSSStyleDeclaration::namedPropertyGetterCustom()
as v8SetReturnValueString() will already take care of returning an empty String if the
input String is null.

R=haraken@chromium.org

Review URL: https://codereview.chromium.org/293003003

git-svn-id: svn://svn.chromium.org/blink/trunk@174395 bbb929c8-8fbe-4397-9dbb-9b2b20218538