[repo-assist] Treat missing dependencies as no dependencies (with test)

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This PR replicates the changes from #4321 (which is from a fork and couldn't be updated directly) and adds the requested test, per @dsyme's request: "Please make a new PR with the same contents as this PR but adding a test."

Root cause

Some OData feeds (e.g. JFrog Artifactory) omit the <d:Dependencies> node entirely for packages that have no dependencies, instead of including it with empty content. NuGetV2.fs previously treated a missing node as an error (failwithf "unable to find dependencies for package..."), causing package resolution to fail against such feeds.

Fix

Cherry-picked from the original PR (commit d98e779ea): when the Dependencies node is absent, treat it as an empty dependency list ("") instead of throwing.

Test added

can parse package without Dependencies node as no dependencies in tests/Paket.Tests/NuGetOData/ODataSpecs.fs, using a new fixture NuGetOData/NoDependenciesNode.xml (an OData entry without the <d:Dependencies> element, modeled on the Artifactory response shape). It asserts the parsed entry has an empty SerializedDependencies list instead of throwing.

Test Status

Ran dotnet test tests/Paket.Tests/Paket.Tests.fsproj --filter "FullyQualifiedName~ODataSpecs" — all 13 tests pass on net9. The net461 target could not execute in this sandbox (Could not find 'mono' host), which is an environment limitation unrelated to this change.

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