Weaken a CHECK to a DCHECK in TemplateURLData to match existing DCHECKs

Some clients have empty keyword urls. This was previously being checked
by a DCHECK that seemingly never hit (maybe it never happened on Canary)

This DCHECK was implicitly hardened to a CHECK in crrev.com/c/6040381 so
this CL weakens it back to a DCHECK to match existing code.

Future CLs will examine whether it's best to drop these invalid records,
but this CL is designed to be safely mergeable.

BUG=393390112

Change-Id: I487fab5ffe7a7c516c951cbda11bca93145fb59f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6219388
Reviewed-by: Tomasz Wiszkowski <ender@google.com>
Commit-Queue: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1413689}