Running Buffer with a distance of zero is not valid, as the expected output would be an exact duplicate of the input.
Recheck the buffer distance entered and rerun the tool with a different value. If you are using a field to set the buffer distance, ensure that all records for that field have a value greater or less than 0. If you did not actually use a value of 0, check that the value you entered is valid. For instance, a value of - 2 that includes a space between the minus sign and the number 2 is validated to 0. For negative buffer distances, ensure that there is no space between the minus sign and the actual number.