Naturally, Looker returns the rows of data in your model that match you filtered criteria. No rows, will result in no data shown as you are experiencing.
Depending on your data, providing you can write SQL to produce the result you want (try starting with a full date table list and left joining / counting your measure from there) then you could use a derived table to meet your specific need. Derived tables should be used sparingly, as they negative the benefits of modelling in LookML, but if its for a single / small number of visuals, they can be structured to achieve your required output