Span builder should accept null in asChildOf

[pavolloffay]

From OpenTracing javadoc:

* If parent==null, this is a noop.

[codecov-io]

Codecov Report

Merging #243 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #243      +/-   ##
============================================
- Coverage     82.02%   81.97%   -0.05%     
+ Complexity      526      524       -2     
============================================
  Files            87       87              
  Lines          2003     2003              
  Branches        236      237       +1     
============================================
- Hits           1643     1642       -1     
  Misses          262      262              
- Partials         98       99       +1

Impacted Files	Coverage Δ	Complexity Δ
...ger-core/src/main/java/com/uber/jaeger/Tracer.java	86.4% <100%> (+0.87%)	18 <0> (ø)	⬇️
...jaeger/reporters/protocols/ThriftUdpTransport.java	77.96% <0%> (-5.09%)	14% <0%> (-2%)

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[jpkrohling]

Is there a way to test this ?

[pavolloffay]

@jpkrohling there is always a way to test it ;)

[pavolloffay]

Can somebody merge this please?