Fix @uplink.*Map to not throw error if converter is None

[prkumar]

Fixes #221

Attention: @prkumar

[codecov]

Codecov Report

Merging #231 (6e24e55) into master (d3ee283) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #231   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          44       44           
  Lines        2398     2400    +2     
  Branches      182      183    +1     
=======================================
+ Hits         2396     2398    +2     
  Misses          2        2           

Impacted Files	Coverage Δ
uplink/converters/keys.py	100.00% <100.00%> (ø)

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