A non existing sale or a sale with a 0 revenue ?

Who does not want to distinguish between a non existing event - sale for instance - and an existing event with a 0 as a numeric result, a revenue due to an promotional discount, or an exceptional 0 cost, etc. ?

Your data path may look like :

Data Source -> Data Mart -> SSAS Cube -> Client Reporting tool (SSRS, Excel)

The absence of data has to be transferred on the whole path from the Data Source to the Client.
Simply put, do not make 0's or ' ' out of the absence of data neither in DataMart, SSAS cube or at the report layer.

Your 'false friends' in this process may be :

1) Lack of awareness of this issue for at least one person in the normal chain :
ETL developer -> SSAS developer -> Client developer.

2) Using functions such as isnull() or coalesce() in the ETL processes. I still found a misconception even among skilled database developers, that avoid null values at the datamart level, as they are 'difficult to tackle in t-sql calculations'. 

3) The default SSAS Nullprocessing option at the measure level is set to Automatic. A better alternative is the Preserve in my opinion. You may recall, that Preserve will preserve the NULL value, the automatic is equivalent to ZeroorBlank option. It converts the null value to zero for numerical columns or to a blank string for text-based columns. 

4) You are not safe at the client level. Your Excel olap pivot table may be set to show 0 for empty cells, or you may use SSRS expressions, even a simple division by 2 for fields values may  accidentally create false 0's.