Added null checks before adding JSON elements to crash reports

[robert-smartbear]

Goal

Prevent rare issues from happening when a crash report is created while some data is not available (e.g. when Bugsnag is not fully initialized)

[github-actions]

Bugsnag.framework binary size did not change - 718,176 bytes

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%      +8  +0.0%      +8    __TEXT,__text
  -0.1%      -8  -0.1%      -8    [__TEXT]
  [ = ]       0  [ = ]       0    TOTAL

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