How to get results with zero count from this query?

Hello, I have a database table that contains information about the timestamp
and location(latitude and longtitude) of the requests made by users. The
column structure is as following:

requesttime(which is a timestamp without time zone type) | latitude |
longtitude

I have written the following code in order to retrieve the total requests
made day by day and hour interval by hour interval starting from a given
datetime to another given datetime:

SELECT date_trunc('hour', requesttime), COUNT(requesttime)
FROM mytable
WHERE requesttime between '2001-04-02 03:12:45' and '2006-02-05 23:14:00'
GROUP BY date_trunc('hour', requesttime)
ORDER BY date_trunc('hour', requesttime);

(a sample result from this query is 2003-07-11 21:00:00 | 121, meaning that
121 requests were made during the 21:00 - 22:00 hourly interval on july 11
2003)

however, I realized that this query skips printing out the time intervals
that have 0 requests -e.g. 2002-03-12 03:00:00 (the 03:00 - 04:00 am
interval) has 0 counts of requests but it directly skips printing that and
prints the 04:00-05:00 interval instead-

How can I make this query to also print out the rows with 0 counts in
addition to the original results? Thanks in advance.

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