Remove empty strings from LHR

[exterkamp]

What is the current behavior?

We should remove empty strings from the LHR to make it strongly consistent with proto + not waste space. e.g.

{
    "displayValue": "",
}

becomes
{}

[exterkamp]

This seemingly isn't too much of an issue. Mostly just a case of the roundtrip testing comparing correctly. We can modify the test to include empty string values in the round trip json, which will make the tests pass all the same but the round trip will be longer. But is totally arbitrary. Personally I think we can just ignore this. @paulirish? @brendankenny? @patrickhulce? Thoughts?