fix: check relationships indexed access for undefined

[wkillerud]

Description

After a change of relationTo in a relationship field, an existing global from before the change caused a crash in the admin UI. The whole page turned blank, and the console logged a TypeError pointing to relationMap[relation].push, where relationMap[relation] was undefined.

TypeScript does not flag undefined as a possibility in these scenarios unless both strict and noUncheckedIndexedAccess are turned on in the compiler options.

I tried to add undefined explicitly in the type definition for RelationMap, but it had no effect.

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Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have added tests that prove my fix is effective or that my feature works
• Existing test suite passes locally with my changes
• I have made corresponding changes to the documentation