OT Trace propagator fails with exception when no headers are present

[marcinzaremba]

Describe your environment
Latest version (main branch) of both opentelemetry-python and opentelemetry-python-contrib

Steps to reproduce

from opentelemetry.propagators.ot_trace import OTTracePropagator
from opentelemetry.propagators.textmap import DictGetter

getter = DictGetter()
carrier = {}

propagator = OTTracePropagator()
propagator.extract(getter, carrier)

results in:

Traceback (most recent call last):
  File "repro.py", line 8, in <module>
    propagator.extract(getter, {})
  File "(...)/opentelemetry-python-contrib/propagator/opentelemetry-propagator-ot-trace/src/opentelemetry/propagators/ot_trace/__init__.py", line 76, in extract
    and _valid_extract_traceid.fullmatch(traceid) is not None
TypeError: expected string or bytes-like object

What is the expected behavior?
Propagator extraction returns an unmodified context due to lack of relevant headers in the carrier.

If a value can not be parsed from the carrier, for a cross-cutting concern, the implementation MUST NOT throw an exception and MUST NOT store a new value in the Context, in order to preserve any previously existing valid value.

from OTel specification

What is the actual behavior?
Propagator extraction fails with an exception when no headers are present in the carrier.

Additional context
N/A