Error on header value of empty string when it isn't required

1. Header param isn't required in openapi spec.
2. Header value is set but the value is empty string.
3. Generated code reports an error `Invalid format for parameter <param_name>: parameter '<param_name>' is empty, can't bind its value` because of [this code](https://github.com/deepmap/oapi-codegen/blob/master/pkg/runtime/bindparam.go#L45) in `BindStyledParameterWithLocation`.