issue_486

[Abigael-JT]

• Removed null assignment and redundant casting
• Replaced commented out code in if statement with an exception
• Returned NullNode instead of null
• Added an exception to avoid a Null pointer exception if aggregation column is null

[elliVM]

Left some comments

[Abigael-JT]

changes made

[51-code]

Left some comments about the exceptions.

[Abigael-JT]

Removed unnecessary check and clarified exception message if the given command is not an aggregation function

[51-code]

Looks good!

[kortemik]

catch of a NullPointerException is not allowable, use null check and throw a proper exception in case no argument is provided. i.e. IllegalArgumentException("required instruction x not provided");

[Abigael-JT]

removed the try catch block and replaced it with null check that throws an exception in eventstats instructions are missing

[Abigael-JT]

typo fixed

[Abigael-JT]

rebased