Change an Emitter log from error to debug

[mhadam]

https://github.com/snowplow/snowplow-android-tracker/blob/master/snowplow-tracker/src/main/java/com/snowplowanalytics/snowplow/tracker/Emitter.java#L410

[AlexBenny]

Update link to code:

snowplow-android-tracker/snowplow-tracker/src/main/java/com/snowplowanalytics/snowplow/internal/emitter/Emitter.java

Line 466 in 50e104b

Logger.e(TAG, "Emitter database empty: " + emptyCount);

It is tracked as an error but it should be a debug log.