tracing: avoid returning None if OpenTelemetry is not installed; instead return a custom noop-span

[odeke-em]

Trying to uniformly instrument this library is really tricky in the case that OpenTelemetry is not installed.
Currently the code returns None in the case that the dependency wasn't imported.

Trying to invoke

with trace_call('foo') as span:
    if span:
         # Add code

and then in other cases where we need to add span annotations firstly having to check if span is not None is awkward such as in #1207

if span is not None:
    span.add_event(...)

we shouldn't labor ourselves with dependency implementation details which constitutes a leaky abstraction.

Suggestion

Let's implement a NoopSpan class that'll be returned on every invocation if not installed and that should make it clean for this package to freely add events.

/cc @harshachinta @surbhigarg92