Skip deleted apps when updating dag_registry

[marioncottard]

Airflow keeps in the database the dags that have been deleted, so this query:
model_dags = [ dag for dag in session.query(DagModel) for tag in dag.tags if tag.name == "model" ] returns deleted dags, which causes the update_dag_registry to fail

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 8 lines in your changes missing coverage. Please review.

Please upload report for BASE (develop@161e0d4). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #588   +/-   ##
==========================================
  Coverage           ?   75.36%           
==========================================
  Files              ?      258           
  Lines              ?    14490           
  Branches           ?     2004           
==========================================
  Hits               ?    10921           
  Misses             ?     3115           
  Partials           ?      454           

Flag	Coverage Δ
dags-tests	77.79% <20.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
server	77.89% <0.00%> (?)
client	78.13% <0.00%> (?)
dags	77.79% <0.00%> (?)

The merge-base changed after approval.