Fix null reference in PaymentDetails validation

This fixes UBSan errors like the following:
https://luci-milo.appspot.com/ui/inv/build-8763888193215235281/test-results?q=PaymentRequestUpdateWithTest.UpdateWithModifiers&sortby=&groupby=

All the validation methods took a const PaymentItem& total parameter and
did nothing with it. But as references cannot be null, the call sites
called it with *details.total, which is undefined if the pointer is
null.

Looking at the history, the bug was introduced in
https://crrev.com/c/643768. In the old version of the file, the
parameter was const payments::mojom::PaymentItemPtr& total, which *can*
be null since it's a reference to a smart pointer. In the conversion,
this was mistranslated to a non-nullable type:

https://chromium-review.googlesource.com/c/chromium/src/+/643768/12/components/payments/content/payment_details_validation.cc
https://chromium-review.googlesource.com/c/chromium/src/+/643768/12/components/payments/core/payment_details_validation.cc

We just didn't notice because the parameter was unused. The last time
the parameter was used was just before this CL:
https://codereview.chromium.org/2852363002

Bug: 1394755
Change-Id: I4b983f4ef2d7e04a1d0eb257dfd82994c95fed50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5046277
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Auto-Submit: David Benjamin <davidben@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1227410}