[mime] Fix the use of MIME images without an explicit encoding

The Content-Transfer-Encoding header is optional: if not present then
the default "7bit" encoding should be assumed.  iPXE already includes
logic to set a default encoding name, but the default encoding name
then fails to match against any entries in the known encodings list
since it is terminated with a NUL (rather than with the semicolon or
whitespace character that would terminate the encoding name found
within a Content-Transfer-Encoding header).

Fix by removing the default encoding name and instead treating a NULL
encoding name as indicating that the default encoding should be used,
and add a test case that omits the Content-Transfer-Encoding header.

Reported-by: Huzaifa Ali Zar <zar@amazon.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>