Option to disable Zero-filling of aggregateBy

**Problem**
The MapAggregator class provides zero-filled results. This may generate a lot of undesired "zero" data entries.

**Describe the solution you'd like**
Zero-filling should be optional or disableable by a flag. For example:

```java
mapaggregator.zerofill(false)
```

**Additional context**
When working with multiple `aggregateBy` steps, and/or relatively sparse datasets (e.g. aggregation by user ids), it's not uncommon that most entries in the zero-filled result are zero. In such cases, these zero entries are typically not of interest, and could in worst case be a significant waste of CPU time and memory.