Fix MongoBackend.clear_db()

[clenk]

so it won't try to delete nonexistent databases

[codecov-commenter]

Codecov Report

Merging #166 (0093c9c) into master (2a588aa) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   95.09%   95.12%   +0.03%     
==========================================
  Files          24       24              
  Lines        2955     2954       -1     
==========================================
  Hits         2810     2810              
+ Misses        145      144       -1     

Impacted Files	Coverage Δ
medallion/backends/mongodb_backend.py	91.94% <100.00%> (+0.25%)	:arrow_up:

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