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.

[changeset-bot]

🦋 Changeset detected

Latest commit: ea48919

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 32 packages

Name	Type
@module-federation/rspack	Patch
@module-federation/enhanced	Patch
@module-federation/modern-js	Patch
@module-federation/nextjs-mf	Patch
@module-federation/node	Patch
@module-federation/rsbuild-plugin	Patch
@module-federation/storybook-addon	Patch
@module-federation/modernjsapp	Patch
remote5	Patch
@module-federation/runtime	Patch
@module-federation/webpack-bundler-runtime	Patch
@module-federation/sdk	Patch
@module-federation/runtime-tools	Patch
@module-federation/managers	Patch
@module-federation/manifest	Patch
@module-federation/dts-plugin	Patch
@module-federation/third-party-dts-extractor	Patch
@module-federation/devtools	Patch
@module-federation/bridge-react	Patch
@module-federation/bridge-vue3	Patch
@module-federation/bridge-shared	Patch
@module-federation/bridge-react-webpack-plugin	Patch
@module-federation/retry-plugin	Patch
@module-federation/data-prefetch	Patch
@module-federation/error-codes	Patch
@module-federation/inject-external-runtime-core-plugin	Patch
@module-federation/runtime-core	Patch
create-module-federation	Patch
@module-federation/cli	Patch
@module-federation/esbuild	Patch
@module-federation/utilities	Patch
website-new	Patch

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[jbroma]

@2heal1 can we get this merged?

[ScriptedAlchemy]

Yes sorry, had some CI failures, let me see if it works now then ill merge this weekend

[jbroma]

bumping this for visibility - anything stopping us from merging this?