extract_histogram_data is broken Enumerable not implemented for nil of type Atom

[bunnymatic]

In a recent mod, it appears that extract_histogram_data no longer works. When I run tests (on a project that includes mogrify), I get the following error

     ** (Protocol.UndefinedError) protocol Enumerable not implemented for nil of type Atom. This protocol is implemented for the following type(s): HashSet, Range, Map, Function, List, Stream, Date.Range, HashDict, GenEvent.Stream, MapSet, File.Stream, IO.Stream
     code: assert @fixture |> ImageProcessor.Impl.compute_theme(:simple_sort) ==
     stacktrace:
       (elixir 1.11.2) lib/enum.ex:1: Enumerable.impl_for!/1
       (elixir 1.11.2) lib/enum.ex:141: Enumerable.reduce/3
       (elixir 1.11.2) lib/enum.ex:3461: Enum.map/2
       (mogrify 0.8.0) lib/mogrify.ex:127: Mogrify.extract_histogram_data/1
...

I'm going to look into the issue and hopefully can have a PR up soon to address this.

I wanted to just document the fact that it's out there in case someone else has noticed issues.

[bunnymatic]

This appears to be related to histogram entries from ImageMagick like

    339: (3.53463,133.003,221.133) #0485DD srgb(1.38613%,52.1579%,86.7188%)

I think the regex is not matching the % signs or the decimals in the rgb values

[bunnymatic]

Closed with #94