codegen_main emit empty schedule proto for combinational design

It seems that `codegen_main` now returns an empty `output_schedule_path` when being called with `--generator=combinational`:
```
$ bazel-bin/xls/dslx/ir_convert/ir_converter_main --top=main xls/examples/crc32.x | bazel-bin/xls/tools/opt_main - | bazel-bin/xls/tools/codegen_main --generator=combinational --delay_model=unit --output_schedule_path=/tmp/schedule.proto - > /dev/null ; stat /tmp/schedule.proto
File: /tmp/schedule.proto
Size: 0 Blocks: 0 IO Block: 4096 regular empty file
Device: 0,51 Inode: 34587305 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 1000/ proppy) Gid: ( 1000/ proppy)
Access: 2024-02-28 14:56:31.442252568 +0100
Modify: 2024-02-28 14:58:33.622256316 +0100
Change: 2024-02-28 14:58:33.622256316 +0100
Birth: 2024-02-28 14:56:26.091252386 +0100
```
It would be nice to return a single stage schedule instead to allow other tools to inspect the critical path of combinational design.