Add spec for continuing tracing with empty TraceDigest (#2905)

* Spec for Tracer#continue_trace! with empty digest

* WIP

* Revert "WIP"

This reverts commit b0c8c19.

spec/datadog/tracing/tracer_spec.rb

@@ -815,6 +815,49 @@
       end
     end
 
+    context 'given empty TraceDigest' do
+      let(:digest) { Datadog::Tracing::TraceDigest.new }
+
+      before { continue_trace! }
+
+      it 'starts a new trace' do
+        tracer.trace('operation') do |span, trace|
+          expect(trace).to have_attributes(
+            origin: nil,
+            sampling_priority: 1
+          )
+
+          expect(span).to have_attributes(
+            parent_id: 0,
+            span_id: a_kind_of(Integer),
+            trace_id: a_kind_of(Integer)
+          )
+        end
+
+        expect(tracer.active_trace).to be nil
+      end
+
+      context 'and a block' do
+        it do
+          expect { |b| tracer.continue_trace!(digest, &b) }
+            .to yield_control
+        end
+
+        it 'restores the original active trace afterwards' do
+          tracer.continue_trace!(digest)
+          original_trace = tracer.active_trace
+          expect(original_trace).to be_a_kind_of(Datadog::Tracing::TraceOperation)
+
+          tracer.continue_trace!(digest) do
+            expect(tracer.active_trace).to be_a_kind_of(Datadog::Tracing::TraceOperation)
+            expect(tracer.active_trace).to_not be original_trace
+          end
+
+          expect(tracer.active_trace).to be original_trace
+        end
+      end
+    end
+
     context 'given a TraceDigest' do
       let(:digest) do
         Datadog::Tracing::TraceDigest.new(