[MRG] update output from gather w/o abundances, so that abund output is empty

[ctb]

Based on dib-lab/genome-grist#45, there is a potential confusion where sourmash gather outputs CSVs that have zeros in three columns (average_abund, median_abund, std_abund) when NOT using abundances in calculations.

This PR changes output CSVs so that None is used as default value, which results in empty CSV fields. I think this is much clearer than putting a zero in the field!

Addresses part of dib-lab/genome-grist#45 (comment)

TODO:

• tests for new behavior

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1328 (05d902d) into latest (128e50c) will increase coverage by 5.33%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           latest    #1328      +/-   ##
==========================================
+ Coverage   88.73%   94.06%   +5.33%     
==========================================
  Files         123       96      -27     
  Lines       18116    14509    -3607     
  Branches     1398     1399       +1     
==========================================
- Hits        16075    13648    -2427     
+ Misses       1802      622    -1180     
  Partials      239      239              

Flag	Coverage Δ
python	94.06% <92.30%> (-0.01%)	⬇️
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/commands.py	82.22% <50.00%> (-0.18%)	⬇️
src/sourmash/search.py	91.15% <100.00%> (ø)
tests/test_sourmash.py	99.41% <100.00%> (+<0.01%)	⬆️
src/core/src/sketch/hyperloglog/estimators.rs
src/core/src/index/sbt/mhbt.rs
src/core/tests/minhash.rs
src/core/src/index/sbt/mod.rs
src/core/src/ffi/cmd/compute.rs
src/core/src/sketch/nodegraph.rs
src/core/src/cmd.rs
... and 20 more

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[ctb]

Ready for review & merge @luizirber!

[ctb]

@bluegenes could review it also, if she's feeling like it ;)

[bluegenes]

Nice - None/ empty columns are much clearer than zeroes!

lgtm

[ctb]

thank you!