[Filestore] TReadDataActor should not memset data that is going to be read, to avoid double writing to memory

Getting rid of unnecessary memset for most workloads, for example:

<img width="2400" height="1118" alt="Image" src="https://github.com/user-attachments/assets/70aa14c1-b820-4437-a1c6-0c669d21ce64" />