v1.tagValues has different output depending on whether the tag values rule is used or not

The `v1.tagValues` call gets a different result depending on whether the push down rule is used.

```
import "influxdata/influxdb/v1"
import "planner"

option planner.disablePhysicalRules = [
"PushDownReadTagValuesRule"
]

v1.tagValues(bucket: "telegraf", tag: "blah")
```

The above will return one row with a null or empty string value. If I re-enable the rule, I get no rows.

Start by reproducing the v1.tagValues call with planner.disablePhysicalRules set to PushDownReadTagValuesRule, then compare the results with the rule enabled. Trace v1.tagValues and PushDownReadTagValuesRule to identify why one path emits a null or empty value while the other emits no rows; done means both paths have consistent output.

Written by the indexing model from the issue text.