fix(rspack): avoid falsy values in checkSingleton

[jbroma]

Description

added a check for falsy values inside checkSingleton in packages/rspack/src/ModuleFederationPlugin.ts.

This is similar to other places in the codebase when looking for ModuleFederationPlugin instances, this specific change was based on the logic from packages/enhanced/src/lib/container/ContainerPlugin.ts

Related Issue

n/a

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the documentation.

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Latest commit: 5395d95

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