After installing botbuilder-core last release from pip, I had the exact error described and fixed in the issue #1880. It appears that the telemetry_logger_middleware.py file hasn't been updated in pip.
It can easily be checked by installing botbuilder-core with pip, then in botbuilder/core/telemetry_logger_middleware.py L220 should be "if activity.attachments and len(activity.attachments) > 0:" instead of "if activity.attachments and activity.attachments.strip():".
Would appreciate if you could update the pip repository.