[AIRFLOW-4131] Make template undefined behavior configurable.

[jmcarp]

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[codecov-io]

Codecov Report

Merging #4951 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4951      +/-   ##
==========================================
- Coverage    75.6%   75.59%   -0.01%     
==========================================
  Files         454      454              
  Lines       29227    29228       +1     
==========================================
  Hits        22096    22096              
- Misses       7131     7132       +1

Impacted Files	Coverage Δ
airflow/models/__init__.py	92.88% <100%> (-0.05%)	⬇️

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[Fokko]

Looking good @jmcarp I like to set this to strict to fail fast :-)

[kaxil]

Please correct me if I am wrong, this is a breaking change, isn't it ??

Behavior before this PR is if the variable is not defined it would error.
But after merging this PR: it would not raise an error.

@jmcarp @Fokko

[Fokko]

The default is Undefined: https://github.com/pallets/jinja/blob/master/jinja2/environment.py#L279

This is still the case if it isn't passed explicitly: https://github.com/apache/airflow/pull/4951/files/65af90c017022c2fb31b24d336e0de45934120cd#diff-664e0133fa172b6e0060f2dd7b2c3504R2990

[kaxil]

Oh yes, you are right... my bad :)
Thanks @Fokko

[Fokko]

No problem, any time @kaxil 👍