Improve compatibility with MLT's new profile (de)serialization.
A null profile prevents MLT from serializing profile information.
There was something strange happening when switching profiles where it
would apply the previous profile's frame rate to the new profile.
There is still a framerate-related bug when you close Kdenlive with a project
profile whose frame rate is different than the default, re-open Kdenlive,
and then use the default profile!
projects / kdenlive / commit