Fix queries without arguments

[nathanjcochran]

It seems that #63 introduced a bug that broke queries that aren't using arguments. Any queries without arguments now return the following error:

InvalidParameter: 1 validation error(s) found. - minimum field size of 1, StartQueryExecutionInput.ExecutionParameters.

The problem is that the buildExecutionParams function returns an empty but non-nil slice when no args are provided. That ultimately gets passed as the ExecutionParameters in the call to StartQueryExecution. That in turn causes a validation error in the underlying Athena library, because it validates that the slice has a length of at least 1 if it's non-nil. See the min:"1" type:"list" struct tag on the ExecutionParameters field in the StartQueryExecutionInput struct.

This fixes the issue by returning a nil slice instead of an empty slice if no args are provided.

[CLAassistant]

All committers have signed the CLA.