Mqtt: reset full/empty times

[andig]

Fix #28811

[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• Changing Full/Empty from *time.Time to time.Time removes the explicit nil/no-value state; double-check all callers and MQTT consumers correctly interpret the zero time.Time and that this doesn’t introduce ambiguity with legitimate time.Time{} values.
• The struct tags json:"full,omitzero" / json:"empty,omitzero" are non-standard for Go’s encoding/json (which only understands omitempty); confirm that your JSON marshaller actually supports omitzero or adjust the tags to match the serializer being used.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Changing `Full`/`Empty` from `*time.Time` to `time.Time` removes the explicit nil/no-value state; double-check all callers and MQTT consumers correctly interpret the zero `time.Time` and that this doesn’t introduce ambiguity with legitimate `time.Time{}` values.
- The struct tags `json:"full,omitzero"` / `json:"empty,omitzero"` are non-standard for Go’s `encoding/json` (which only understands `omitempty`); confirm that your JSON marshaller actually supports `omitzero` or adjust the tags to match the serializer being used.

## Individual Comments

### Comment 1
<location path="core/site_optimizer.go" line_range="73-74" />
<code_context>
 	batteryDetail
-	Full  *time.Time `json:"full"`
-	Empty *time.Time `json:"empty"`
+	Full  time.Time `json:"full,omitzero"`
+	Empty time.Time `json:"empty,omitzero"`
 }

</code_context>
<issue_to_address>
**issue (bug_risk):** Using `omitzero` in JSON tags is non-standard and likely ignored by `encoding/json`

`encoding/json` only supports the `omitempty` option. With `json:"full,omitzero"`, `Full` will still always be marshaled, and a zero `time.Time` becomes `"0001-01-01T00:00:00Z"`. If you need to omit zero timestamps, you’ll need a custom `MarshalJSON` (on `batteryResult` or a wrapper `time.Time` type) rather than relying on this tag option.
</issue_to_address>

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[sourcery-ai]

issue (bug_risk): Using omitzero in JSON tags is non-standard and likely ignored by encoding/json

encoding/json only supports the omitempty option. With json:"full,omitzero", Full will still always be marshaled, and a zero time.Time becomes "0001-01-01T00:00:00Z". If you need to omit zero timestamps, you’ll need a custom MarshalJSON (on batteryResult or a wrapper time.Time type) rather than relying on this tag option.