Adobe Javascript If/Else Statements with empty cells

The basic Boolean logical operator for "Or"  is " || ". And also, when you are using a comparison operator with a logical statement to determine a difference in values (or  between variables)  you should employ "!==" ( not equal value or not equal type with the double equal comparison operator).

Last, in your last line,  you're declaring the null statement for the event value with " " (a spcae in between the quotes); the quotes should be together "". Otherwise, you're declaring a statement based on a value that corresponds to a single space character string inside of the quotes.

Is there a specific reason why yo needed to declare the event value like this?

I would deploy your script like this:

var B8 = this.getField("B8").value;
var B9 = this.getField("B9").value;
var C5 = this.getField("C5").value;
var C6 = this.getField("C6").value;
var C7 = this.getField("C7").value;
var C8 = this.getField("C8").value;

var GOAL = C7+(B9-(C8-B8))+C5-C6+25;

event.value = "";

if ( ( C5 !== "" ) || ( C6 !== "" ) || ( C7 !== "" ) || ( C8 !== "" ) ) {event.value = GOAL;}