Native messaging hosts module assumed all hosts are packages, and we
were passing null before.
The patch also adds a test case for a null firefox package to avoid
regressions in the future for this common (on Darwin at least) scenario.
Note: Thunderbird doesn't need a similar change because it doesn't allow
a null package.
Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>
