fix: accept /dev/null as input

[Yongcheng123]

Summary

• fall back to a non-epoll cancel reader when WithInput is /dev/null
• keep the fallback narrowly scoped to the specific epoll setup failure from issue The input isn't verified before using epoll on it #964
• add a regression test covering WithInput(os.Open(os.DevNull))

Testing

• go test ./... -run TestTeaProgramRunsWithDevNullInput
• go test ./...
• go vet ./...

Closes #964