[0.6] Make header recording null safe (#158)

[felixbarny]

Backports the following commits to 0.6:

• Make header recording null safe (Make header recording null safe #158)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (0.6@85bc5f3). Click here to learn what that means.
The diff coverage is 40%.

@@         Coverage Diff          @@
##             0.6   #159   +/-   ##
====================================
  Coverage       ?    77%           
  Complexity     ?    957           
====================================
  Files          ?     99           
  Lines          ?   3301           
  Branches       ?    293           
====================================
  Hits           ?   2542           
  Misses         ?    602           
  Partials       ?    157

Impacted Files	Coverage Δ	Complexity Δ
.../java/co/elastic/apm/servlet/ServletApiAdvice.java	5.12% <0%> (ø)	1 <0> (?)
...main/java/co/elastic/apm/impl/context/Request.java	72.88% <100%> (ø)	20 <2> (?)
...lastic/apm/report/serialize/DslJsonSerializer.java	84.68% <66.66%> (ø)	85 <0> (?)

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