zq: Call user-defined op with source

[mattnibs]

The change allows zq users to start a query with a call to a user-defined op that contains a source (file) with no additional input sources. Previously attempting to do this would result in error "redundant inputs".

Fixes #4701

[philrz]

Testing this branch at commit 777dc58, it seems to address #4701. The final command shown below now returns the expected output rather than the redundant inputs error shown in #4701.

$ zq -version
Version: v1.10.0-11-g777dc583

$ cat file_in_op_no_call.zed
op CallMe(): (
  file sample.zng | count()
)

$ zq -I file_in_op_no_call.zed 'CallMe()'
31(uint64)

That makes it a functional 👍 for me. I'll mark it in the opening text as "Fixes #4701" so the issue will close when this merges.