Fix blank dag dependencies view

[bbovenzi]

When there is an issue with the scheduler max_last_updated may be older than last_refresh and therefore calculate_graph was never being called leaving the nodes and edges to be blank. Removing it allows us to calculate the nodes and edges even in that circumstance.

Update: I added a check to calculate the graph when there isn't already node or edge data.

Bonus: Make the last_refresh change with the app's timezone like all other dates.

Closes #16610

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[bbovenzi]

P.S If this makes sense then we probably should remove the class method SerializedDAG.get_max_last_updated_datetime entirely. And I'm not sure if we should even bother with the last_refresh logic since I'm not sure when we would want to return empty data instead of trying to calculate the graph.

[ashb]

I would guess that the thinking here is that "if none of the serialized dags have changed, then there is no point recalculating the graph"

[bbovenzi]

CC: @ms32035

[bbovenzi]

Ok. Instead, I updated to calculate the graph when there isn't already node or edge data.

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.