For unmapped histogram fields we instantiate by default a DoubleHistogram
with 3 significant digits precision. The test generates a random value
for the number of significant digits that is in the range [0, 5].
As a result, if the test runs with 4 or 5 significant value digits but
the HdrHistogram sketch only uses 3, checking errors on results will fail.

Here we change the maximum value for the significant value digits to 3
if the query involves an index with unmapped fields.

Normallt we would create a histogram with the correct number of digits,
nut for empty histograms we end up serializing and deserializing large
arrays for empty aggregations. Here we have a kind of workaroud were we
use just 3 digits for empty histograms and, at reduce time, we always
merge usign the larger number of digits among all histograms.

When an empty aggregations uses a TDigest object the udnelrying
arrays used by the AvlTree TDigest implementation is eagerly
allocated. If the aggreation produces no result, we serialize and
deserialize an array which might be large if the comrpession value
is large (about 5 * compression centroids are tracked). Here we
use a null value for empty aggreations while building the result
and later on use a static empty TDigest object at reduce time and
merge it with the non-empty results.

This change will end up in another PR focused on improving
memory usage for TDigest percentiles aggregations.