bug when setting $_POST array field to null?

I have some array fields in my html ie:
<input type="text" name="sold_for_price[]">
<input type="text" name="sold_for_price[]">
<input type="text" name="sold_for_price[]">
etc
which is not a mandatory fields, which  means the first one can be 12.00, second one can be empty, third one can be $1.25 etc

so when submitting to post it becomes:

$_POST['sold_for_price'] = array('12.00', '', '1.25');

however in mysql, money type cannot be empty string, so I'll have to manually convert it into null. ie:

if (strlen($_POST['sold_for_price'][i]) == 0) $_POST['sold_for_price'][i] = null; //if field is not entered, I'll change the value to null

however, changes won't reflect in $this->input->post('sold_for_price')[1]

but if I do: $_POST['some_other_money_field'] = null, then $this->input->post('some_other_money_field') will be null.

PHP Code:

$_POST['sold_for_price'] = array('12.00', '0.00', '1.25'); 


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