fix: do not inject invalid span context

[dyladan]

Fixes #2066

[codecov]

Codecov Report

Merging #2080 (35a69b8) into main (a457776) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head 35a69b8 differs from pull request most recent head a93aed9. Consider uploading reports for the commit a93aed9 to get more accurate results

@@            Coverage Diff             @@
##             main    #2080      +/-   ##
==========================================
- Coverage   93.03%   92.98%   -0.06%     
==========================================
  Files         154      154              
  Lines        5976     5976              
  Branches     1246     1246              
==========================================
- Hits         5560     5557       -3     
- Misses        416      419       +3     

Impacted Files	Coverage Δ
...y-core/src/context/propagation/HttpTraceContext.ts	100.00% <100.00%> (ø)
...etry-exporter-prometheus/src/PrometheusExporter.ts	90.66% <0.00%> (-1.34%)	⬇️
...ges/opentelemetry-instrumentation-http/src/http.ts	94.82% <0.00%> (-0.80%)	⬇️