Calculate Percentage with JOINS and original value
One way to get expected output can be
select case when total_count = 0 then 0.0 else count_1*100.0/total_count end as pcnt 
from 
(
SELECT count(case when b.statition_id is null then 1 end) as count_1, 
count(*) as total_count 
FROM `bigquery-public-data.new_york_citibike.citibike_trips` as a
LEFT OUTER JOIN  `bigquery-public-data.new_york_citibike.citibike_stations` as b 
ON a.start_station_id = b.station_id
)x 

Note, it returns 0.0 if total_count is 0 to avoid division by zero.  You may want to return null(casted to proper decimal type ) in such case instead.