subscriber: skip padding when skipping `log.*` fields in `DefaultVisitor`

[Porges]

Motivation

Closes #2979.

The current behaviour of DefaultVisitor is that it will write padding even if it is going to skip writing a value, which results in extraneous padding being added when values are skipped by the tracing-log integration.

Solution

With this change, DefaultVisitor will only insert padding if it is actually going to write a value.

[taikulawo]

same here

[xxchan]

Hi @hds, it's been a while since tracing-log 2.0. Would you mind publishing a new version?