Tool should be able to handle null column data and treat it as an empty array

[nthitz]

Currently the filter tool breaks if column data has metadata: null things break. This should be treated as metadata: [] and just treat the file as if columns have no metadata

[iimog]

I have a simple (but not perfect) fix in my pull request #67, see https://github.com/PitchInteractiveInc/Phinch/pull/67/files#diff-3283ec1d765c474b09e8714d4a319672R98
This fix only works if the metadata is null for the first row.
As you can check at https://blackbird.iimog.org the file from issue #82 loads without any problems there.

If you decide using the biojs-io-biom javascript library (disclosure: I am an author of this library) as is also part of my pull request #67 this conversion could also be handled there.
If you have ideas for improvement of this library (or its documentation) let me know.