Fix tagging with empty data

[TonyCTHsu]

fixes #3088

What does this PR do?

An object includes Tagging module, but #tags method raise NoMethodError: undefined method 'merge' for nil:NilClass or TypeError: no implicit conversion of nil into Hash

Because the instance variables are nil.

The fix is to replace instance variable with protected attribute reader.

[codecov-commenter]

Codecov Report

Merging #3102 (8131af7) into master (0318c77) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3102   +/-   ##
=======================================
  Coverage   98.15%   98.15%           
=======================================
  Files        1323     1323           
  Lines       75085    75094    +9     
  Branches     3421     3421           
=======================================
+ Hits        73702    73711    +9     
  Misses       1383     1383           

Files Changed	Coverage Δ
lib/datadog/tracing/metadata/tagging.rb	97.61% <100.00%> (ø)
spec/datadog/tracing/metadata/tagging_spec.rb	100.00% <100.00%> (ø)

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