[Merge M73][NTP] Fix console error if no suggestions are available

If no suggestions are available run the callback with a string setting
search_suggestions.suggestionHtml to an empty string rather than nullptr.
This fixes the console error:
'search-suggestions.js:1 Failed to load resource: net::ERR_FAILED'.

Bug: 904565
Change-Id: I9671c2e39297bf10802f1d2feacff6fab2e6d06b
Reviewed-on: https://chromium-review.googlesource.com/c/1446789
Reviewed-by: Ramya Nagarajan <ramyan@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#627691}(cherry picked from commit 0e82675707acbcafd5cba04efdbe04efabfd5cfd)
Reviewed-on: https://chromium-review.googlesource.com/c/1450302
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/branch-heads/3683@{#122}
Cr-Branched-From: e51029943e0a38dd794b73caaf6373d5496ae783-refs/heads/master@{#625896}
1 file changed