fix: ignore invalid json responses

[nlf]

We have seen occasional responses for codecov badges returning svg even though we've requested json, this is intended to ignore non-json responses and move on anyway. I chose to return null since that's what is currently being done for non-string responses, which this is the same class of failure as.

[codecov]

Codecov Report

Merging #29 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #29   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           6      6           
  Lines          58     61    +3     
  Branches       16     17    +1     
=====================================
+ Hits           58     61    +3

Impacted Files	Coverage Δ
lib/util/shieldsIo.js	100% <100%> (ø)	⬆️

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