Tweak the implementation of scan_apply to match the python client's d…

[hawka]

…ocumentation, wherein providing additional args to a UDF is optional.

Ref: Documentation for Client.scan_apply

In the current implementation, passing no py_args into the Client.scan_apply() function results in a segfault, which is no bueno.

If the ability to pass no py_args is not the desired behavior, then I believe that the documentation should be changed, and line 103 modified to also ensure that py_args is present and return a useful error:

if (!(namespace_p) || !(py_set) || !(py_module) || !(py_function) || !(py_args)) {
    as_error_update(&err, AEROSPIKE_ERR_PARAM, "Parameter should not be null");
    goto CLEANUP;
}

[jboone100]

Thanks @hawka ! This will be included in the 2.0.2 release.