* refactor: default empty `if:` statement to `if: success()` in evaluator
Previously the code to default an empty `if:` statement in the yaml file
was implemented in different files in the model package.
Now an empty `if:` statement defaults to `success()` in the expression
evaluator.
* refactor: remove obsolete `FixIfStatement` functions
The introduction of the expression evaluator seems to have made these
functions obsolete, as the test case `TestRunEvent/issue-598` works even
without these functions.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
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