Backmerge: #1912 Support unresolved IDT monomers (no structure, IDT only) (#1988)

[AliaksandrDziarkach]

Backmerge from master to 1.21

Backmerge request

• [ ] PR name follows the pattern Backmerge: #1234 – issue name
• [ ] PR is linked with the issue
• [ ] base branch (master or release/xx) is correct
• [ ] code contains only backmerge changes