Resolve "Cannot add External or Prediction"

Description

• Fixed writing undefined into cerebral state at multiple places
• Covered external and prediction - are there more? "addReferences" seems to be a candidate
• Needs to be carefully reviewed!! This has potential to mess up the DB!!
• TODO?: Prune annotation object for properties containing null before POST to customannotations/? Example: annotations: {prediction: {ortholog_conservation: null, dna_conservation: null}} 0627ad6b

Related issues

Closes #1907 (closed)

Notes to review (code/docs/QA)

Tests

General

• Tests have been added that prove my fix is effective or that my feature works
• Related tests have been modified/removed

Hypothesis testing:

• Soak testing has been done
• Distribution between positive / negative cases has been checked

Database

• Includes changes to database schema
• Includes necessary database migrations

Configuration

• Includes changes to configuration
• Includes configuration migration instructions in documentation

Merge checklist

• Self-review of code has been performed.
• Feature review and validation against specification has been performed (if applicable). Apply label: QAdone
• Need for documentation has been evaluated and, if necessary, updated. Apply label: docsdone
• Code and implementation has been reviewed by other core developer (including any changes based on initial review). Apply label: code reviewdone