Fix destroy function to prevent null error during Pie chart animation

[vtange]

addresses #2187
Modified .destroy function to not simply set $$.config and $$.data to null, as during a pie animation, these will quickly be checked as d3 animates things frame by frame. I set config back to the default, and data to an object with an empty targets array.

[codecov-io]

Codecov Report

Merging #2188 into master will decrease coverage by 0.1%.
The diff coverage is 10%.

@@            Coverage Diff             @@
##           master    #2188      +/-   ##
==========================================
- Coverage   73.88%   73.77%   -0.11%     
==========================================
  Files          51       51              
  Lines        4185     4191       +6     
==========================================
  Hits         3092     3092              
- Misses       1093     1099       +6

Impacted Files	Coverage Δ
src/interaction.js	35.22% <0%> (ø)	⬆️
src/api.chart.js	10% <0%> (-2.5%)	⬇️
src/arc.js	69.54% <100%> (ø)	⬆️

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