Form 61A is a filing requirement under Rule 114E(1) of the Income Tax Rules. It deals with "Statement of Financial Transaction"
http://incometaxindia.gov.in/Rules/Income-Tax%20Rules/103120000000007545.htm
Form 61B flows from Rules 114G(8) and deals with FATCA/ CRS reporting
http://www.incometaxindia.gov.in/Rules/Income-Tax%20Rules/ITR114g.htm
There is no mention of Nil Return in Rule 114E. So, filing of Nil return of 61A seems to not required.
But, As per 114G(8), we need to file Nil returns. So, filing 61B Nil return is compulsory.