Ensure that conversions from/to 8bit and 16bit never return null string.

WTF::StringImpl::upconvertedString() relies on make16BitFrom8BitSource()
return a valid (non-null) string, on which releaseImpl() can be called.
The patch ensures that for zero length sources an empty string is
returned. The same logic is applied to make8BitFrom16BitSource().

R=abarth@chromium.org
BUG=409977

Review URL: https://codereview.chromium.org/531913002

git-svn-id: svn://svn.chromium.org/blink/trunk@181263 bbb929c8-8fbe-4397-9dbb-9b2b20218538