Set SpanContext on SentryTransaction to avoid potential NPE.

[maciejwalkowiak]

📜 Description

Set trace context on SentryTransaction and pass it to contexts on transaction#finish.

💡 Motivation and Context

The alternative to #1164

💚 How did you test it?

📝 Checklist

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

[maciejwalkowiak]

@bruno-garcia @marandaneto I think this is a better than #1164 approach - the result the discussion we had. I think trace context on finish is cleaner than doing it during serialization.

[bruno-garcia]

Much nicer!