[ECalBackendEws] Prevent crash when item has no MIME content set
[ECalBackendEws] Prevent crash when item has no MIME content set

There could happen that the item on the server didn't have set any MIME
content, in which case the code could dereference a NULL pointer (in libical).

This had been reported downstream as:
https://retrace.fedoraproject.org/faf/reports/1916092/

GNOME/evolution-ews,GNOME/evolution-ews