fix: handle change to collectDoubles in convertTileDriftSeriesInfoToTimeSeries (#293)

## Summary
[Recent
changes](01a2f70#diff-deee840c61f04e77fc1a50be94e1e153c193fb4b00a7b1389a733e0941c29714)
to `collectDoubles` in PivotUtils now return null instead of empty lists
for invalid data. This broke the /timeseries endpoints which assumed
non-null lists.

Updated TimeSeriesHandler to:
- Safely handle null lists using Option
- Return empty sequences instead of failing
- Fix isNumeric checks to handle null percentileDriftSeries

@sean-zlai to test:
You can check that the charts are broken on main, due to /timeseries
being broken. If you check out this branch and run everything from
scratch (`./docker-init/build.sh --all`), it should fix the charts.

## Checklist
- [ ] Added Unit Tests
- [ ] Covered by existing CI
- [ ] Integration tested
- [ ] Documentation update



<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Bug Fixes**
	- Improved null handling in time series processing
	- Enhanced robustness of data conversion methods
	- Reduced risk of potential runtime exceptions

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: ken-zlai

hub/src/main/scala/ai/chronon/hub/handlers/TimeSeriesHandler.scala

@@ -119,14 +119,17 @@ class TimeSeriesHandler(driftStore: DriftStore) {
     // check if we have a numeric / categorical feature. If the percentile drift series has non-null doubles
     // then we have a numeric feature at hand
     val isNumeric =
-      tileDriftSeries.percentileDriftSeries.asScala != null && tileDriftSeries.percentileDriftSeries.asScala
-        .exists(_ != null)
+      Option(tileDriftSeries.percentileDriftSeries).exists(series => series.asScala.exists(_ != null))
+
     val lhsList = if (metric == NullMetric) {
-      tileDriftSeries.nullRatioChangePercentSeries.asScala
+      Option(tileDriftSeries.nullRatioChangePercentSeries).map(_.asScala).getOrElse(Seq.empty)
     } else {
-      if (isNumeric) tileDriftSeries.percentileDriftSeries.asScala
-      else tileDriftSeries.histogramDriftSeries.asScala
+      if (isNumeric)
+        Option(tileDriftSeries.percentileDriftSeries).map(_.asScala).getOrElse(Seq.empty)
+      else
+        Option(tileDriftSeries.histogramDriftSeries).map(_.asScala).getOrElse(Seq.empty)
     }
+
     val points = lhsList.zip(tileDriftSeries.timestamps.asScala).map {
       case (v, ts) => TimeSeriesPoint(v, ts)
     }