Remove unnecessary null check in ConnectorStream.split(enumClass, splitter)

There are unnecessary null check in split method of ConnectorStream.java

I thought there are posibillity to return a null in getEnumConstants() so created null check logic.

public <E extends Enum<E>> EnumMap<E,TStream<T>> split(Class<E> enumClass, Function<T, E> splitter) {

        E[] es = enumClass.getEnumConstants();
        if(es == null) {
            throw new IllegalArgumentException("Class object does not represent an enum type");
        }

But the argument defined that enumClass is alway enum<E>. It's my mistake. The null checker is not necessary.

It's simple change but create a jira issue because I believe it is important to leave a record of the changes and mapping 1-1(or 1-N) to github PR.