Change Samplers to return null instead of provided attributes

[Nevay]

Returned attributes are additional attributes; AlwaysOffSampler already returns null.
"A set of span Attributes that will also be added to the Span."

[codecov]

Codecov Report

Merging #713 (b1dddc6) into main (e728c53) will decrease coverage by 1.04%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main     #713      +/-   ##
============================================
- Coverage     83.82%   82.77%   -1.05%     
- Complexity     1233     1243      +10     
============================================
  Files           138      139       +1     
  Lines          2980     3037      +57     
============================================
+ Hits           2498     2514      +16     
- Misses          482      523      +41     

Flag	Coverage Δ
7.4	82.77% <100.00%> (-1.06%)	⬇️
8.0	82.82% <100.00%> (-1.05%)	⬇️
8.1	82.82% <100.00%> (-1.05%)	⬇️

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Impacted Files	Coverage Δ
src/SDK/Trace/Sampler/AlwaysOnSampler.php	100.00% <ø> (ø)
src/SDK/Trace/Sampler/TraceIdRatioBasedSampler.php	100.00% <100.00%> (ø)
src/API/Trace/SpanContext.php	82.92% <0.00%> (-4.26%)	⬇️
src/API/Trace/TraceState.php	100.00% <0.00%> (ø)
src/SDK/Common/Attribute/Attributes.php	100.00% <0.00%> (ø)
...c/API/Trace/Propagation/TraceContextPropagator.php	100.00% <0.00%> (ø)
src/SDK/Common/Util/TracingUtil.php
...ation/SanitizeCombinedHeadersPropagationGetter.php	0.00% <0.00%> (ø)
src/SDK/Common/Exception/StackTraceFormatter.php	0.00% <0.00%> (ø)
src/SDK/Trace/RandomIdGenerator.php	85.71% <0.00%> (+5.71%)	⬆️

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