Avoid giving GMime a NULL MIME-stream filter.

Avoid giving GMime a NULL MIME-stream filter.

Micah Anderson reported an issue where a message failed to display in
the emacs interface, (it instead gave an error, "json-read-string: Bad
string format").

Micah tracked this down to the json output from "notmuch show" being
interrupted by a GMime error message:

	gmime-CRITICAL **: g_mime_stream_filter_add: assertion
	`GMIME_IS_FILTER (filter)

I tracked this down further to notmuch passing a NULL value to
g_mime_stream_filter_add. And this was due to calling
g_mime_filter_charset_new with a value of "unknown-8bit".

So we add a test message withe a Conten-Type of "text/plain;
charset=unknown-8bit" from Micah's message. Then we fix "notmuch show"
to test for NULL before calling g_mime_stream_filter_add. Bug fixed.