Specify the key for 'null or empty value detected' payload log (close #277)

[Buck Ryan (b-ryan)]

There's no way to know which key is causing the "null or empty value detected" log, so this change makes this log possible to debug.

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[Buck Ryan (b-ryan)]

I signed it

[Snowplow CLA bot (snowplowcla)]

Confirmed! Buck Ryan (@b-ryan) has signed the Contributor License Agreement. Thanks so much.

[Paul Boocock (paulboocock)]

Thanks for the PR Buck Ryan (@b-ryan)

I'm looking at doing a small release in february so I'll include this then.

[Miranda Wilson (mscwilson)]

Thanks again for this PR Buck Ryan (@b-ryan), and sorry for the delay in merging it in. We'll be releasing v0.11.0 including your change soon.