Fixing RemoteBaggageRestrictionManager NullPointerException on empty response

[ravirajj]

Which problem is this PR solving?

• RemoteBaggageRestrictionManager throws NullPointerException #630
• Received the following exception when the RemoteBaggageRestrictionManager attempted to update baggage restrictions:

Exception in thread "Timer-1" java.lang.NullPointerException
    at io.jaegertracing.internal.baggage.RemoteBaggageRestrictionManager.updateBaggageRestrictions(RemoteBaggageRestrictionManager.java:93)
    at io.jaegertracing.internal.baggage.RemoteBaggageRestrictionManager.updateBaggageRestrictions(RemoteBaggageRestrictionManager.java:87)
    at io.jaegertracing.internal.baggage.RemoteBaggageRestrictionManager$1.run(RemoteBaggageRestrictionManager.java:67)
    at java.util.TimerThread.mainLoop(Timer.java:555)
    at java.util.TimerThread.run(Timer.java:505)

• The restrictions response may be null because gson.fromJson() returns null when HttpBaggageRestrictionManagerProxy receives an empty response from the remote server.

Short description of the changes

• Throw BaggageRestrictionManagerException if the restrictions response is empty.

[codecov]

Codecov Report

Merging #631 into master will increase coverage by 0.1%.
The diff coverage is 50%.

@@             Coverage Diff             @@
##             master     #631     +/-   ##
===========================================
+ Coverage     89.51%   89.62%   +0.1%     
- Complexity      561      564      +3     
===========================================
  Files            69       69             
  Lines          2061     2063      +2     
  Branches        261      262      +1     
===========================================
+ Hits           1845     1849      +4     
+ Misses          136      134      -2     
  Partials         80       80

Impacted Files	Coverage Δ	Complexity Δ
...ernal/baggage/RemoteBaggageRestrictionManager.java	94.91% <50%> (-5.09%)	11 <0> (+1)
...gertracing/internal/reporters/LoggingReporter.java	90.9% <0%> (+9.09%)	5% <0%> (+1%)	⬆️
...rtracing/internal/reporters/CompositeReporter.java	100% <0%> (+28.57%)	7% <0%> (+1%)	⬆️

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