Use safe navigation operator to avoid GraphQL errors when querying nonexistent package

What does this MR do and why?

This MR is intended to address #434001. I assembled this during the FY25-Q1 Support Hackathon: gitlab-com/support/support-team-meta#5827 (closed).

This MR adds a single character. In PackageDetailsResolver, this line is adjusted so that GraphQL doesn't return an error when someone attempts to query a package that doesn't exist.

Before

        package if package.default?

After

        package if package&.default?

MR acceptance checklist

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Before this change

After this change

How to set up and validate locally

To observe the current behavior, browse to gitlab.com/-/graphql-explorer and issue the query below.

To observe my changes:

1. Browse to https://gdk:3000/-/graphql-explorer
2. Issue a query like the one below
3. Observe the Internal server error in the message that is returned

query package {
  package(id: "gid://gitlab/Packages::Package/00099999999999999999999999") {
    id
    name
  }
}