fix(dynamic-sampling): Do not freeze DSC if no Sentry values are present

[tonyo]

From https://develop.sentry.dev/sdk/performance/dynamic-sampling-context/#freezing-dynamic-sampling-context:

When an SDK receives an HTTP request that was "instrumented" or "traced" by a Sentry SDK, the receiving SDK should consider the incoming DSC as instantly frozen. Any values on the DSC should be propagated "as is" - this includes values like "environment" or "release".

SDKs should recognize incoming requests as "instrumented" or "traced" when at least one of the following applies:
The incoming request has a sentry-trace header
The incoming request has a baggage header containing one or more keys starting with "sentry-"

Therefore, when creating a DSC from a header, let's freeze it only when there's at least one sentry- value.

[codecov]

Codecov Report

Base: 83.75% // Head: 83.86% // Increases project coverage by +0.11% 🎉

Coverage data is based on head (ade23ec) compared to base (fe08ff2).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head ade23ec differs from pull request most recent head 25bddf6. Consider uploading reports for the commit 25bddf6 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #532      +/-   ##
==========================================
+ Coverage   83.75%   83.86%   +0.11%     
==========================================
  Files          23       23              
  Lines        2801     2802       +1     
==========================================
+ Hits         2346     2350       +4     
+ Misses        374      370       -4     
- Partials       81       82       +1     

Impacted Files	Coverage Δ
dynamic_sampling_context.go	93.42% <100.00%> (+4.08%)	⬆️
tracing.go	82.26% <100.00%> (+1.72%)	⬆️
integrations.go	89.01% <0.00%> (-2.32%)	⬇️
transport.go	78.37% <0.00%> (-0.85%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.