EntityHydrator crashes if a parent relation is null

[jnugh]

Describe the bug
Let's say we have three entities (ProductVariant, MetaEntity, Configuration). MetaEntity is a nullable customField of ProductVariant and Configuration is related to MetaEntity.

If I use the EntityHydrator to fetch Configuration I get an error in getRelationEntityAtPath at

const target = parent[part];

because ProductVariant.customFields.metaEntity was null parent['configuration'] is not accessible.
If I understand correctly it would be okay to just return null here as nothing needs to be translated or calculated.

To Reproduce
Define a nullable custom field relation which contains another relation. Hydrate the nested relation while the first one is null.

Expected behavior
It should return null for the top level relation.

Environment (please complete the following information):

• @vendure/core version: latest (master)
• Nodejs version: 20
• Database (mysql/postgres etc): postgres / sqlite

Additional context
I already implemented a fix for this, will create a PR as well.