Rack: Fix missing active trace

[TonyCTHsu]

What does this PR do?

closes #3389

When tracing and distributed_tracing both disabled, The reference of Datadog::Tracing.active_trace in rack
instrumentation returns nil.

Causing

NoMethodError: undefined method `resource_override?' for nil:NilClass

[p-datadog]

The exception happens in the call to set_request_tags! made in this method. Wouldn't it be better to expand the condition on line 72 to check if tracing is disabled, and if so, skip the remainder of the method?

[TonyCTHsu]

👋 @p-datadog , your suggested alternative will definitely also mitigate this error.

The reason for my change is because it follows the same null object pattern from our API .trace(..) when invoke with a block (Yeah, it is kinda flawed that the blockless form only return a span_operation, hence the trace_operation is only accessible through .active_trace )

I think the best solution for this would be to instrument with a block, this would by default yield null objects for both span_operation and trace_operation, that would made all those conditionals obsolete.

However, that is a much bigger change for this already messy instrumentation.