Issue with decrement variable

[jcuribe]

Using PHP 7.1 when using variableDecrement and Increment an exception is thrown.

We have traced the error back to the ezcWorkflowNodeArithmeticBase::Execute.

The issue arises because VariableDecrement and Increment do not set $configuration/operand, therefore at the moment when execute checks if it's set an exception is thrown.

The correction is to modify ezcWorkflowNodeArithmeticBase::Execute,
add line 100> $operand = 1;
add line 115> if ($operand !==null) { $this->operand = $operand }
else