Empty prefix alias for wildcards aggregate functions

Moving and downsampling data between databases or retention policies should allow us to maintain the field names.

I tried

```
SELECT last(*) AS ""
INTO "mfcf_vmware"."autogen".:measurement
FROM /^vmware_.*/
WHERE time >= '2016-01-01T00:00:00Z'
GROUP BY time(30m), "host"
```

but sadly because the prefix key is empty it fallbacks to "last_" in this case.

https://github.com/influxdata/influxdb/pull/7009
@jsternberg would adding this be simple?
@gunnaraasen should something different be done so it uses backreferences?

Start by reading the discussion and linked pull request #7009, then reproduce the shown SELECT last(*) query against wildcard measurements. Determine how an empty prefix is handled for wildcard aggregate aliases; done means the query preserves the original field names instead of falling back to names beginning with "last_".

Written by the indexing model from the issue text.