Resolve "Variants/ROI tracks broken for empty analyses"

Description

• return [] instead of None from get_alleles_from_db o avoid additional logic in callers for empty allele lists
• fix for parsing url args when no allele IDs are given

Related issues

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

Closes #2115 (closed)