How to use the @liskhq/lisk-validator.isPositiveNumberString function in @liskhq/lisk-validator

public constructor(rawTransaction: unknown) {
		super(rawTransaction);
		const tx = (typeof rawTransaction === 'object' && rawTransaction !== null
			? rawTransaction
			: {}) as Partial;
		// Initializes to empty object if it doesn't exist
		if (tx.asset) {
			const rawAsset = tx.asset as RawAsset;
			this.asset = {
				data: rawAsset.data,
				recipientId: rawAsset.recipientId,
				amount: new BigNum(
					isPositiveNumberString(rawAsset.amount) ? rawAsset.amount : '0',
				),
			};
		} else {
			// tslint:disable-next-line no-object-literal-type-assertion
			this.asset = {
				amount: new BigNum('0'),
				recipientId: '',
			} as TransferAsset;
		}
	}