Update javadoc to allow null/empty attr values

[breedx-splk]

Resolves #5501.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.02 ⚠️

Comparison is base (9b9efa6) 91.31% compared to head (93600c6) 91.30%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5616      +/-   ##
============================================
- Coverage     91.31%   91.30%   -0.02%     
+ Complexity     4982     4981       -1     
============================================
  Files           554      554              
  Lines         14749    14749              
  Branches       1376     1376              
============================================
- Hits          13468    13466       -2     
- Misses          887      888       +1     
- Partials        394      395       +1     

Impacted Files	Coverage Δ
...src/main/java/io/opentelemetry/api/trace/Span.java	93.47% <ø> (ø)

... and 3 files with indirect coverage changes

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