Crash-loop detection fix: zero restarts wrongly transmitted

v0.17.1-beta

Crash-loop detection fix: zero restarts wrongly transmitted. The restart counter (restarts) introduced in 0.17.0-beta was omitted from the JSON when its value was 0 (the default behaviour of omitempty on an integer), whereas a freshly (re)created container reports precisely 0 in a meaningful way. The hub read that case as "agent too old, no data available" instead of "genuinely zero restarts", which could skew crash-loop detection right after a deployment. The field is now always serialised.