Add Explicit Return Types and Discussion on Return Value

I have added return types to all the files.
This issue is now open for discussion regarding the return value of functions like getBalance(). Should they return 0 or NULL?
cc: @anybody

Let's first fix 🐛 Fix tests Active until we proceed here.

getBalance() effectively cannot return null due to several constraints:

The balance field is defined as required in the Giftcard entity:

$fields['balance'] = BaseFieldDefinition::create('commerce_price')
  ->setLabel(t('Balance'))
  ->setDescription(t('Current balance'))
  ->setRequired(TRUE)

Entity validation enforces this. In GiftcardTest::testSaving() an entity without a balance fails validation.

All code paths that create gift cards set a balance:

• Direct creation sets a balance
• Gift card purchases set a balance
• Bulk generation sets a balance

The Redemption pane has a check for null when it probably should be checking for zero

$balance = $giftcard->getBalance();
if (!$balance) {
  $form_state->setError($pane_form, $this->t('Unable to check gift card %code balance, please try again.', ['%code' => $giftcard->getCode()]));
}

Becomes

if ($giftcard->getBalance()->isZero()) {
  $form_state->setError($pane_form, $this->t('The provided gift card %code has no balance.', ['%code' => $giftcard->getCode()]));
}

Thanks @rhovland!

@lavanyatalwar are you planning to proceed on this?

Sure, wroking on it.

Done with the changes
@anybody, @rhovland Kindly check and merge :)