Null-safe share-scope merge

[Jaouan]

Description

Fixes a bug occurring when loading a VitePluginFederation remote in an application that mixes ModuleFederation and VitePluginFederation remotes.

When a remote ModuleFederation is already present in the share scope, a loading: null property is set (source).
During share scope merging, since typeof null === "object", the call to Object.assign(null, …) throws the following error: Cannot convert undefined or null to object.

This fix adds a null check to prevent calling Object.assign on a null value.

(maybe it is recommended to have a dedicated scope for this kind of case?)

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Code of Conduct and follow the Commit Convention guidelines.
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• Ideally, include relevant tests that fail without this PR but pass with it.