KafkaBatchSliceCollection dereferencing a pointer that might be null

**Describe the bug**
`processRange()` method calls `generate()` with offsets that might be null. 
 
**Expected behavior**
Check If either offsets are null do not generate batch slices if so, instead return an empty collection.
 
**Software version**
3.3.4
 
**Additional context**
Coverity: 1594360 Dereference null return value 
