fix: Guard Object.assign against null/undefined resourceUris (CLARK-CLIENT-Q)

[sentry]

This PR addresses TypeError: Object.assign requires that input parameter not be null or undefined (CLARK-CLIENT-Q) occurring in /onion/dashboard/.

Root Cause:
The error occurred in src/app/core/learning-object-module/learning-object/learning-object.service.ts around line 145. When a learning object was malformed (e.g., after a 404 response from POST /learning-objects//bundle due to an empty object ID), entity.resourceUris would be undefined. The subsequent call to Object.assign(entity.resourceUris) would then throw a TypeError because Object.assign was called with null or undefined as its first argument, and there was no guard to prevent this.

Changes Made:

1. Added undefined guard: The code block responsible for iterating and fetching resource URIs (lines 145-164) is now wrapped in an if (entity.resourceUris !== undefined) condition. This prevents Object.assign from being called with an undefined value and skips resource fetching for malformed learning objects.
2. Corrected Object.assign usage: Changed Object.assign(entity.resourceUris) to Object.assign({}, entity.resourceUris) to ensure a proper shallow clone of the resourceUris object is created, aligning with the likely original intent.

These changes are consistent with existing null/undefined handling patterns found elsewhere in the same file (e.g., lines 235-243).

Fixes CLARK-CLIENT-Q

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