Investigate KafkaBatchSliceCollection class null offsets

[Abigael-JT]

Description

• Investigate the null offsets and test null offsets given by spark framework
• Find a way to make kafka results immutable and therefore making the class immutable

For more context see comments in this pull request and issue

• Fix KafkaBatchSliceCollection dereferencing a pointer that might be null #183
• KafkaBatchSliceCollection dereferencing a pointer that might be null #182

Below PR will make the kafka query offsets immutable and could be possibly used to replace the offsets from the processRange() method

• Refactor planner Kafka Query #191