Improve deadline diagnostics for null DagRun fields

[fat-catTW]

Why

DagRun-based deadline references can evaluate to None when the DagRun exists but the referenced timestamp column is unset, such as logical_date on manually triggered runs without -l.

Before this change, Airflow treated that None the same as a missing DagRun and logged a misleading “Could not find DagRun” warning. Deadline creation was then skipped without logging the actual reason, making the configuration hard to diagnose.

relates: #71750

Solution

Distinguish a missing DagRun row from a present DagRun with a NULL referenced column.

When deadline creation is skipped because the reference evaluates to None, log structured context including the Dag id, run id, deadline alert id, reference type, and the DagRun column required by built-in references.

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[ColtenOuO]

Nice work! Thanks for your fix!

I noticed an edge case and noted it in the comment :D

[ColtenOuO]

Thanks! well done :D

[ferruzzi]

Can you add a 1-line bugfix newsfragment? Other than that I think this LGTM and I want to get it merged before a handful of others while need to rebase on top of this fix.