You can try this in your method;

if(this.v.global.cash.amount == null){
         this.v.global.cash.amount = 0
    }
if(this.v.global.sec.amount == null){
     
         this.v.global.sec.amount = 0
    }

And then you can do that; (your method)

calculatePercents() {
this.v.global.total.amount = this.v.global.cash.amount + this.v.global.sec.amount;
console.log("Cash => " + JSON.stringify(this.v.global.total.amount));
console.log('------------------------');
this.v.global.cash.percent = (this.v.global.cash.amount / (this.v.global.cash.amount + this.v.global.sec.amount)) * 100;
console.log(" Global cash " + JSON.stringify(this.v.global.cash.percent));

this.v.global.sec.percent = (this.v.global.sec.amount / (this.v.global.cash.amount + this.v.global.sec.amount)) * 100;
console.log(" NULL ???? ==> " + JSON.stringify(this.v.global.sec.percent));

var totPercent = (this.v.global.cash.percent + this.v.global.sec.percent);

this.v.global.total.percent = totPercent;

}

And your veriable type is can be "let". Simple Code:

let a = null;

if(a==null){
   a=0
}
console.log(a);
// output: 0

with || if the first value is false (converted as boolean) then it will take the second, in that case it's used as a default value.

this.v.global.total.percent = totPercent || 0;

you can use || operator to set zero

Eg:

this.v.global.total.percent = (totPercent||0);

for more https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR