Fix exporting spans with links

[indrekj]

This fixes exporting spans that include links. Previously these resulted
in this error:

exception error: bad map: {attributes,128,infinity,0,#{}}

The test is not the most sophisticated one, but it at least catches this
particular error.

[codecov]

Codecov Report

Merging #340 (5349796) into main (724460c) will increase coverage by 0.72%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #340      +/-   ##
==========================================
+ Coverage   32.55%   33.27%   +0.72%     
==========================================
  Files          56       56              
  Lines        4559     4559              
==========================================
+ Hits         1484     1517      +33     
+ Misses       3075     3042      -33     

Flag	Coverage Δ
api	67.28% <ø> (ø)
elixir	16.44% <ø> (ø)
erlang	33.21% <ø> (+0.73%)	⬆️
exporter	21.33% <ø> (+1.35%)	⬆️
sdk	77.13% <ø> (+0.25%)	⬆️
zipkin	2.59% <ø> (ø)

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Impacted Files	Coverage Δ
...ntelemetry_exporter/src/opentelemetry_exporter.erl	73.45% <ø> (+0.61%)	⬆️
...er/src/opentelemetry_exporter_trace_service_pb.erl	17.35% <0.00%> (+1.41%)	⬆️
apps/opentelemetry/src/otel_links.erl	73.33% <0.00%> (+13.33%)	⬆️

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