Make "name" an optional argument to track_screen_view

[fblundun]

Currently it is mandatory. This shouldn't be the case: https://github.com/snowplow/iglu-central/blob/master/schemas/com.snowplowanalytics.snowplow/screen_view/jsonschema/1-0-0
See snowplow/snowplow-java-tracker#63