Fix schedule run value in Source overview and schedule pages

[edmundito]

Change description

This fixes a minor issue primarily seen in ui-config where sometimes the schedule and overview pages would fail if the list of runs was empty because next expects null instead of undefined as the return value.

Checklists

Development

• Application changes have been tested appropriately

Impact

• Code follows company security practices and guidelines
• Security impact of change has been considered
• Performance impact of change has been considered
• Possible migration needs considered (model migrations, config migrations, etc.)

Please explain any security, performance, migration, or other impacts if relevant:

Code review

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• Relevant tags have been added to the PR (bug, enhancement, internal, etc.)