Did you actually look for and find a syringe in the taxi? Did both you and the drive search the back seat thoroughly? If no syringe was found, then there was no syringe.
Most likely there was a broken spring in the back seat which jabbed you or something in your own pocket like a pen or pencil.
You stated your skin was NOT broken and there was NO blood.
Bottom line............you had NO RISK.
Since you have concerns about this, perhaps the next time you use public transportation, check the seat BEFORE you sit in it.