It is ok to Receive Close Tolerance Zero. What is your requirement or issue you are facing.
Eg. If you set RCT as 10% in this case if you receive 90 Unit out of 100 Unit receipt will close.
If Receive Close Tolerance set blank, system consider it is as zero or infinity?
It will be Zero .
It is practical scenario like if you receive Petrol and during transportation some Petrol like 10 lt. out of 100 lt get Vaporised or Wasted so in this case you will receive 90 Lt and close the receipt.
In above case RCT will be 10 % in Oracle.