[iOS] Handle reload transitions with null items in SafeBrowsingTabHelper

It is rare but possible for a WebStatePolicyDecider to receive a
reload transition while both the current and pending navigation
items are null.

While steps to reproduce are not known, these may be cases where
WebKit incorrectly labels a navigation as being of type
WKNavigationTypeReload.

In this situation, SafeBrowsingTabHelper tries to check if the
item being reloaded has an associated UnsafeResource, but this leads
to crash since the item is null.

This CL fixes the crashes by making SafeBrowsingTabHelper check
whether the item is null before checking if it has an UnsafeResource.

Bug: 1208098
Change-Id: Ic5a4bbcd30bc198dccfdf3f5e81b0a87fae10200
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2889676
Commit-Queue: Ali Juma <ajuma@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#882090}