3-Points which supports the viewpoint that HSN Summary filing is not mandatory.
1. Whatever the notifications are available so far mandates accesses having turnover above 1.5 Crs or Exporters to provide HSN on Invoice, else not. It is important to note down that notifications specifically use word "Invoice" and does not use the word "GSTR-1" or "GST Return". So, no notification available/issued so far which mandates filling HSN Summary (Table-12) in GSTR-1. People are liking word "Invoice" with "GSTR-1" implicitly.
2. When you file NON-NIL GSTR-1 and specifically enter sale invoices (B2B, or B2C, or Exports), system does not restrict users from filing return when HSN Summery is not filled. If there is any law which mandates filling HSN Summary in GSTR-1 (though still not available), then it means GST Portal is not abiding the law properly.
3. You can use Tally without inventory. In such scenario, production of HSN Summary is impossible. It is to be noted down that Tally is a registered GST Service Provider. It will not give any feature in the system which does not confirms with GST Law.
But on contrary, it has been witnessed that GST Professionals and even CBIC Officials are delivering speeches or issuing text material which states that HSN Summary filing is mandatory. But this viewpoint is not supported by any law.