[shared storage] Add null check for `worklet_host_` in relevant SharedStorageWorkletDevToolsAgentHost methods

DevToolsAgentHost is refcounted and could outlive the associated
`SharedStorageWorkletHost`. In rare edge situations, crashes can be
triggered due to null `worklet_host_` in `script_source_url()`, etc.

This CL adds the null check and return empty/nullptr for those cases.

Bug: 1469983
Change-Id: Id8ce0481860ae7e7632a15df216098d568ba2169
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4877715
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1198744}