Always draw borders if task instance state is null or undefined - #18033

Task instances of manual runs are usually drawn without borders. However, when a task instance has null state, it blends into the background without a border. In this case, drawing a border makes sense, but draw it at reduced opacity to distinguish it from scheduled runs.

Task instances with undefined state were previously always drawn with full-opacity borders because external_trigger for these task instances was undefined. This has been fixed, so these task instances are now drawn with borders and the proper opacity, depending on whether they are part of manual or scheduled runs.

Common task instance properties are now defined in a single place, and external_trigger is included as a common property so that it will not be undefined as described above.

Fixes #16877.

Screenshots:

1. DAG with one scheduled run and one manual run

2. Clear all task instances in manual run. This results in task instances with null state.

• Before:

• After:

3. Clear all task instances in the scheduled run

• Before:

• After:
  • Still works as expected

4. Add new tasks to the DAG. This results in task instances with undefined state:

• Before:
  • No difference in borders of manual vs. scheduled run

• After
  • Notice difference in border opacity

5. Mouseover effect of the scheduled run

• Before:
  
  

• After:

  • Still works as expected

6. Mouseover effect of the manual run

• Before:
  • The mouseover behavior is inconsistent: one has mouseover effect and the other does not

• After:
  • Both now have no mouseover effect

^ Add meaningful description above

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