Improve ITransaction and ISpan null-safety compatibility

[maciejwalkowiak]

📜 Description

Improve ITransaction and ISpan null-safety compatibility. Also makes contexts on SentryBaseEvent immutable which prevents cases when someone would set context to null on transactions.

💡 Motivation and Context

Fixes #1159

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• All tests passing
• No breaking changes

🔮 Next steps

Since in case of SentryEvents trace can be null, where for SentryTransactions trace cannot be null, I am considering refactoring Contexts class and create separate class for TransactionContext. The drawback would be the code duplication.

[codecov-io]

Codecov Report

Merging #1161 (9dd38ad) into main (8159439) will increase coverage by 0.03%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##               main    #1161      +/-   ##
============================================
+ Coverage     75.25%   75.29%   +0.03%     
+ Complexity     1661     1660       -1     
============================================
  Files           173      173              
  Lines          5808     5800       -8     
  Branches        568      567       -1     
============================================
- Hits           4371     4367       -4     
+ Misses         1173     1170       -3     
+ Partials        264      263       -1     

Impacted Files	Coverage Δ	Complexity Δ
...entry/src/main/java/io/sentry/NoOpTransaction.java	29.16% <0.00%> (+1.16%)	7.00 <0.00> (ø)
sentry/src/main/java/io/sentry/SentryEvent.java	68.75% <ø> (-0.33%)	43.00 <0.00> (ø)
sentry/src/main/java/io/sentry/Span.java	89.47% <ø> (ø)	11.00 <0.00> (ø)
sentry/src/main/java/io/sentry/SpanContext.java	94.11% <ø> (ø)	15.00 <0.00> (ø)
sentry/src/main/java/io/sentry/Hub.java	75.52% <100.00%> (+0.67%)	74.00 <0.00> (ø)
...entry/src/main/java/io/sentry/SentryBaseEvent.java	92.10% <100.00%> (-0.21%)	21.00 <1.00> (-1.00)
...try/src/main/java/io/sentry/SentryTransaction.java	98.21% <100.00%> (-0.07%)	26.00 <0.00> (ø)

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