fix: Only add pre/post context if possible

[HazAT]

No description provided.

[codecov-io]

Codecov Report

Merging #1458 into master will decrease coverage by 7.68%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #1458      +/-   ##
==========================================
- Coverage   79.26%   71.58%   -7.69%     
==========================================
  Files          50       40      -10     
  Lines        2021     1207     -814     
  Branches      466      229     -237     
==========================================
- Hits         1602      864     -738     
+ Misses        408      332      -76     
  Partials       11       11

Impacted Files	Coverage Δ
packages/node/src/parsers.ts	84.94% <77.77%> (-0.61%)	⬇️
packages/raven-node/vendor/json-stringify-safe.js
packages/raven-node/lib/instrumentation/http.js
packages/raven-node/lib/utils.js
packages/raven-node/lib/client.js
...ges/raven-node/lib/instrumentation/instrumentor.js
packages/raven-node/lib/transports.js
packages/raven-node/vendor/node-lsmod.js
packages/raven-node/lib/parsers.js
packages/raven-node/index.js
... and 2 more

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