Custom transaction fails to validate if FEE member is not defined

[lsilvs]

Expected behavior

Expect custom transaction to not crash during validation if FEE member is not defined.

Actual behavior

The PR #3893 introduced a new static member called FEE to BaseTransaction. This property should be implemented by custom transactions to define its own fee but it crashes during validation if it's not defined.

Steps to reproduce

Create a custom transaction extending BaseTransaction like:

class CustomTransaction extends BaseTransaction {
	static get TYPE () {
		return 10;
	}

	validateAsset() {
		return[];
	}
}

Then:

const tx = new CustomTransaction({
  recipientId: '10881167371402274308L',
});

tx.sign('passphrase');
tx.validate();

validate() will throw eq() not a number: undefined

Which version(s) does this affect? (Environment, OS, etc...)

2.1

[lsilvs]

The FEE member can also be used as default fee when instantiating the transaction the same way TYPE is used when type property is not given.