Potential unexpected nulls

[colinscape]

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.

[erickhun]

@colinscape if we had your suggested check, https://github.com/bufferapp/buffer-analytics/pull/524 wouldn't have been necessary :D I'll do a PR with your suggested changes soon

Also related to #10

[colinscape]

@erickhun - ah! 😅 Makes sense! 👍