Fix issue where material instance name cannot be set without creating a default instance

[kunyoungparkk]

This PR is related to PR #8147

After applying PR #8147, FMaterial::createInstance has an issue where it ignores the name passed as a parameter when mDefaultMaterialInstance is not set.

By applying this PR, a material instance can be assigned a name even if mDefaultMaterialInstance is not set.

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[pixelflinger]

@kunyoungparkk would you be able to sign the CLA so we can merge this change? Thank you!

[pixelflinger]

@kunyoungparkk windows build is failing

[kunyoungparkk]

@kunyoungparkk windows build is failing

I’m currently investigating the issue as the build is working fine in my local Windows environment. It might take some time to identify the root cause. I appreciate your patience, and I will provide an update once the issue is resolved.