Use Auth+Capture if upgrading and no option was set

I just noticed that in the function _commerce_stripe_get_txn_capture_bool(), it assumes that a txn_type option will always be set, and returns NULL if no conditions are met. This is not ideal, since the returned value is also submitted to the payment gateway.

/**
 * Used in commerce_stripe_submit_form_submit to get the capture flag for the
 * charge.
 *
 * @param array $payment_method
 *  An array containing the payment_method information.
 * @param array $pane_values
 *  An array containing values from the Commerce Stripe payment pane.
 *
 * @return bool
 *  Returns TRUE if capture is set & FALSE for auth only.
 */
function _commerce_stripe_get_txn_capture_bool($payment_method, $pane_values) {
  $txn_type = $payment_method['settings']['txn_type'];
  // This handles the case when we are in the payment terminal. The
  // $pane_values contains which type of transaction chosen
  if (!empty($pane_values['txn_type'])) {
    $txn_type = $pane_values['txn_type'];
  }

  // The capture flag in the charge takes a boolean. This simply
  // translates the txn constants to a bool.
  if ($txn_type == COMMERCE_CREDIT_AUTH_CAPTURE) {
    return TRUE;
  }
  else {
    if ($txn_type == COMMERCE_CREDIT_AUTH_ONLY) {
      return FALSE;
    }
  }
  return NULL;
}

We should fix this to ensure only a Boolean is ever returned, and also to handle the default if no option was set. (Since Capturing a payment is the default behavior, this makes a sensible default.)