[Payment] Check for null data returned from the web database

Before this patch, a disconnected database or unsuccessful read would
return a null unique_ptr to Web Payments, which did not check for null
pointer. This resulted in occasional null pointer dereference crashes.

This patch checks for null unique_ptr being returned from the web
database to Web Payments. In addition, the unique_ptr is being moved
among methods now, instead of passing around a raw pointer.

After this patch, a disconnected database or unsuccessful read do not
cause a null pointer dereference crash.

Bug: 1473982
Change-Id: I7ff9b26539a3c5207dd95987df3a6c70ba0bf9f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4799470
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1186122}