Logger: ignore empty lines in JSON Mode

[josephjclark]

Short Description

In the logger, when running in json mode (ie, emit JSON objects, not strings), ignore anything with an empty message.

Fixes #899

Implementation Details

This PR is a little misleading because really what it does is prevents the worker sending empty log lines to Lightning.

But the implementation can be handled very neatly by the low level logging component. It'll simply not emit any empty log lines.

This makes sense: json logging is designed for tools to consume logs. And those tools don't need formatting log lines. Usually in the runtime we print empty lines to make the CLI pretty.

Note that console.log(undefined) will still log.

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[josephjclark]

@stuartc this should do you for empty log lines. Do you want to run any QA against this branch or would you rather we deploy to staging to see?