Don't crash by form submission in a document with null encoding.

The CString object which is passed to normalizeLineEndingsToCRLF() can be
a null string. It is created in FormDataList::appendString(), and it
produces a null CString if FormDataList::m_encoding is a null encoding.

BUG=291744,306086
TEST=automated

Review URL: https://codereview.chromium.org/26960002

git-svn-id: svn://svn.chromium.org/blink/trunk@159428 bbb929c8-8fbe-4397-9dbb-9b2b20218538