Potential unexpected nulls #3

In the line

        $ddTraceSpan = \DDTrace\GlobalTracer::get()->getActiveSpan();

I suspect it's possible for this to come back with a null, either in the call to get() or the call to getActiveSpan(), if something hasn't worked quite as expected.

I think we should be more explicit about checking these return values before we use them, else we risk some fatal errors and processes shutting down, just because of the logging - which we want to be as invisible as possible.