[sampler] treat invalid SpanContext as no SpanContext

[thisthat]

Which problem is this PR solving?

• Fixes ParentBasedSampler should not drop invalid SpanContext #2164

Short description of the changes

• Add a check for invalid parent SpanContext
• Add unit tests

[codecov]

Codecov Report

Merging #2165 (d021756) into main (b9c8426) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2165      +/-   ##
==========================================
- Coverage   92.77%   92.75%   -0.03%     
==========================================
  Files         140      140              
  Lines        4997     4997              
  Branches     1028     1028              
==========================================
- Hits         4636     4635       -1     
- Misses        361      362       +1     

Impacted Files	Coverage Δ
...metry-core/src/trace/sampler/ParentBasedSampler.ts	83.87% <100.00%> (ø)
...emetry-core/src/platform/node/RandomIdGenerator.ts	87.50% <0.00%> (-6.25%)	⬇️