Support for 0-D case in Linalg ops

Support for 0-D case in Linalg ops

This CL adds support for O-D ops in Linalg ops by:
1. making the CopyOp maps optional instead of default valued
2. allowing certain map operations to accept and return empty maps
3. making linalg::LowerToLoops aware of these changes
4. providing a proper 0-D impl for CopyOp and FillOp
5. adding the relevant tests

PiperOrigin-RevId: 254381908