#57178: Allow CORS requests with Origin value "null".

Add a new configuration option "cors.allow.nullorigin" for CorsFilter to
explicitly allow requests with Origin value "null" (disabled by
default). When enabled, requests with Origin "null" will be handled as
valid CORS requests and processed by the filter.

See: https://issues.apache.org/bugzilla/show_bug.cgi?id=57178

Signed-off-by: Gregor Zurowski <gregor@zurowski.org>