Fix encoding check for Snowplow events

[adatzer]

Currently in when the advanced option to apply entity settings also to Snowplow events is enabled, in order to decide whether to b64encode the resulting context the logic is meant to be:

• Use the original encoding of the incoming Snowplow tp2 event.
• If this is not possible (i.e. no context or self-describing event), then use the corresponding configuration option.

However there is currently a bug when trying to determine the incoming encoding, as can be seen here (missing check for possibly existing cx field).