Allow empty custom contexts

[BenFradet]

from snowplow/snowplow#2927:

We published javascript-tracker-core 0.4.0 two years ago. Until now it wasn't used in mainstream tracker because included one significant change: it allowed empty custom contexts which would break compatibility with Scala Hadoop Shred < 0.9.14.
But turns out Kinesis Elasticsearch Sink bears this chunk of code and prohibits empty context (and also Scala Analytics SDK.
This all means we should either:
Allow empty contexts here and in Analytics SDK and wait two years more when these versions will be obsolete.
Abandon com.snowplowanalytics.snowplow/contexts/jsonschema/1-0-1 Schema and use old 1-0-0, which makes more sense to me.