PhD application research interest
For the abstract just state you did a masters without a thesis. Universities are used to the fact that not all applicant followed a standard model for their prior studies.
As for research interest I’m unclear what the challenge is? I would pick a topic that is not too general (cell biology) so that the interest is hard to elaborate on or too specific (Actin-related protein 2/3 complex subunit 1B) as to suggest hyper-specialization. Make sure you pick a topic that can be studied in the program you are applying to and explain why it is interesting to you.