request_body_size is dropped from CommandLineOptions when no request header is configured

*Title*: request_body_size is dropped from CommandLineOptions when no request header is configured

*Description*:
`OptionsImpl::toCommandLineOptions()` sets `request_options.request_body_size` inside the loop over the configured request headers, so with `--request-body-size N` but no `--request-header` the size is not emitted into the `CommandLineOptions` proto. Executions routed through the gRPC service (`--nighthawk-service`) therefore run with an empty body. It should always be emitted.

*Reproduction steps*:
```
nighthawk_client --request-body-size 1234 --nighthawk-service http://target/
```
Compare the executed request body size (0) with a run that also passes any `--request-header`.

*Logs*:
No error is logged; the proto simply lacks the field.

*Call Stack*:
Not a crash.

Start with OptionsImpl::toCommandLineOptions(), where request_options.request_body_size is assigned inside the configured request-header loop. Reproduce with --request-body-size 1234 and --nighthawk-service without --request-header, then verify that the CommandLineOptions proto carries the size and the executed request is no longer empty.

Written by the indexing model from the issue text.