🐛 FIX: MarkdownIt.render; don't copy empty env arg

[hukkin]

This PR makes MarkdownIt.render not copy an env if it is an empty AttrDict, making it possible for a caller to capture changes made to env. This is in line with how MarkdownIt.parse method, and the JS implementation works.

[codecov]

Codecov Report

Merging #77 (ea640bf) into master (b533d6d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #77   +/-   ##
=======================================
  Coverage   95.40%   95.40%           
=======================================
  Files          75       75           
  Lines        3746     3747    +1     
=======================================
+ Hits         3574     3575    +1     
  Misses        172      172           

Flag	Coverage Δ
pytests	95.40% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
markdown_it/main.py	92.30% <100.00%> (+0.06%)	⬆️

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[chrisjsewell]

cheers