[INTERNAL] removed NullSarosSession class

Bogus Eclipse API as its best. While the whole API states that null
values are allowed the WithExpression just fails with variables that are
null.

if (variable == null) {
throw new CoreException(
	new ExpressionStatus(
		ExpressionStatus.VARIABLE_NOT_DEFINED,
		Messages.format(
			ExpressionMessages.WithExpression_variable_not_defined,
			fVariable
		)
	)
);
}

Change-Id: Ie43e6cf163e9029b797142b03631629735996e2e
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/2035
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>