codegen_main emit empty schedule proto for combinational design

[proppy]

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.