[MSG] Handle null fields in temporary states

[sarahscott]

Sometimes Impulse will return a conversation but with certain fields empty (i.e. in the few minutes after you create an inquiry). This results in errors like this in Emission, which expects last_message and initial_message to be non-null:

We should be able to handle these intermediary states and either not return the conversation to a client if it isn't ready or have some sort of 'loading' state.