Failure while Injecting a context

[randomswdev]

Describe the bug
An application using apmot terminates unexpectedly because of a null pointer.

To Reproduce
I'm reproducing this in a complex environment. But looking at the methods Inject and Extract in module/apmot/tracer.go, the extract method returns a SpanContext with the tx field set to nil. On the other side the Inject method uses the value returned by spanContext.Transaction() without checking it. This generates the null pointer dereference.

Expected behavior
The application to run successfully.