Defensively code BabylonParser object accessors

[ripixel]

Summary

Object properties are accessed before the object can be said to exist in a few cases in the BabylonParser, causing fatal JS errors in applications that suffer these bugs. Defensively coding this parser to check for existence before attempting to access properties ensures no JS fatal errors.

Test plan

BabylonParser tests still green post-changes; however, fixes my specific issues while using jest-community/vscode-jest#92 which uses jest-editor-support.

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[codecov-io]

Codecov Report

Merging #3259 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3259   +/-   ##
=======================================
  Coverage   64.24%   64.24%           
=======================================
  Files         175      175           
  Lines        6436     6436           
  Branches        4        4           
=======================================
  Hits         4135     4135           
  Misses       2300     2300           
  Partials        1        1

Impacted Files	Coverage Δ
...s/jest-editor-support/src/parsers/BabylonParser.js	98.94% <100%> (ø)	⬆️

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[aaronabramov]

@orta do you mind looking at this?

[orta]

Yep this looks great 👍

[aaronabramov]

thank you!
@anarresk can you resolve conflicts? :)

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