Private proxy fix

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This PR fixes a missing sanitize method in the PrivateProxyDb

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@stevenyoungs I think we should not allow proxies to return None from the db.get_OBJECT() (or any method) as that hides bugs. It should be the case that sanitizing the object removes all inaccessible handles/references.

[Nick-Hall]

Returning None also leaks the information that the object exists, which is another reason not to do so.

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I've paired down this PR to just fixing bugs in proxies.

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The LivingProxyDb is going to require a rewrite. And I think there is an error when one proxy is on top of another.

I'll go ahead and turn off draft, and do a major rewrite when @stevenyoungs is done.

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@Nick-Hall, this could go into 6.0 as it is just a regular bug fix.

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Originally reported here: https://gramps-project.org/bugs/view.php?id=12886

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When we revise the proxies to make sure they work correctly (sanitize all, proper proxy layering, no need to check for None, etc) we also need to look at caching the results. When I exported 2 people, one person was sanitized 6 times, and the other 3 times. Sanitizing is an expensive process and there is no need to do it more than once.

[I would put this note in a github issue if we turned those on. I see issues as being for developers, not a replacement for mantis].

[stevenyoungs]

The change looks good to me.

Here are some observations from looking at the code around his change.
The implementation of copy_citation_ref_list has the following:

            handle = citation.get_reference_handle()
            source = db.get_source_from_handle(handle)
            if source and not source.get_privacy():
                clean_obj.add_citation(citation_handle)

1. there is no check on handle.
   Is there any way a citation can exist without a reference_handle?
   I know the standard UI forces a source to be selected when creating a citation.
2. if source
   A nice example of a check that can be removed if Minimal code changes to support type hinting of db methods #1934 gets accepted. get_source_from_handle would have raised an error. Even today, it will in most scenarios.
3. if we are prepared to use source.private rather than call source.get_privacy() it can be optimised to get the raw object
   I'm undecided on this.

            data = db.get_raw_source_from_handle(handle)
            if not source.private:
                clean_obj.add_citation(citation_handle)

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I just made this function to work and look exactly like the others. I am re-writing private and living proxies completely so I wouldn't worry about the details of this sanitize function.

[Nick-Hall]

Yes. Keep bug fixes simple.