Servers return wrong column types in case of empty results (everything defaults to STRING)

[sajjad-moradi]

Non-empty results (correct types):

Empty results (incorrect types):

This part of the code needs to be changed:
https://github.com/apache/incubator-pinot/blob/master/pinot-core/src/main/java/org/apache/pinot/core/common/datatable/DataTableUtils.java#L121

[Jackie-Jiang]

This happens when all the segments are pruned out, and Pinot server short-circuits the query to directly return without checking the schema. We should just not return the schema when there is no record returned.