[SPARK-19378][SS] Ensure continuity of stateOperator and eventTime metrics even if there is no new data in trigger

## What changes were proposed in this pull request?

In StructuredStreaming, if a new trigger was skipped because no new data arrived, we suddenly report nothing for the metrics `stateOperator`. We could however easily report the metrics from `lastExecution` to ensure continuity of metrics.

## How was this patch tested?

Regression test in `StreamingQueryStatusAndProgressSuite`

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #16716 from brkyvz/state-agg.