[MRG] test, and actually fix, find-nullomers script; add documentation and test input file

[ctb]

Fixes #1754.

• Is it mergeable?
• make test Did it pass the tests?
• make clean diff-cover If it introduces new functionality in
  scripts/ is it tested?
• make format diff_pylint_report cppcheck doc pydocstyle Is it well
  formatted?
• Did it change the command-line interface? Only backwards-compatible
  additions are allowed without a major version increment. Changing file
  formats also requires a major version number increment.
• For substantial changes or changes to the command-line interface, is it
  documented in CHANGELOG.md? See keepachangelog
  for more details.
• Was a spellchecker run on the source code and documentation after
  changes were made?
• Do the changes respect streaming IO? (Are they
  tested for streaming IO?)

[ctb]

Ready for review & merge.

[ctb]

I super strongly suspect I can get away with a smaller SIZE - SIZE/2, to be specific. Hmm.

[codecov-io]

Codecov Report

Merging #1768 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1768   +/-   ##
======================================
  Coverage    0.05%   0.05%           
======================================
  Files          80      80           
  Lines        9835    9835           
  Branches     2485    2485           
======================================
  Hits            5       5           
  Misses       9830    9830

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