Zero is not the same as a missing value. A missing value indicates something is unknown and it is almost never appropriate to substitute one for the other. Many people substitute missing values with a zero to make their graphs look better or prepare the data for modeling. However, a zero indicates a specific value, while a missing value indicates the value is not known. For instance, when we are tramping the Kepler and are counting the number of people on the track, we would have to leave it blank with the view below. We simply cannot see all of the track, so cannot know how many people are on the track at that moment by visual inspection.
On the other hand, if we are counting the number of people on the deck of the hut below, we can easily see there are zero people.
There are rare times that replacing missing values with zeros might be appropriate. For instance, if your CRM has no pledges for a donor prospect, most reports would by default show this donor's giving as a missing value. However, you know that they have not given anything yet, so replacing their giving amount with a zero on reports is appropriate. However, if you track donors' number of children, replacing a missing value with a zero would not be appropriate. Taking the time to think about each value will help you decide when it is appropriate to replace missing values with zeros (or vice versa).