pdm: tolerate getEffectiveModel during indexation

it can happen that getEffectiveModel does not find a model, this
is currently causing activity failures in indexation of
transformations. Tolerate errors here because this is not an error
but just a sign of invalid user data.
12 jobs for fix/transformation-index-error in 0 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  External
failed ERP5.CodingStyleTest-TestRunner2

01:58:07

failed ERP5.UnitTest-TestRunner2

04:15:41

failed ERP5.UnitTest-Zope2-ForTestRunner2

03:27:09

failed ERP5.CodingStyleTest-TestRunner2

02:24:28

failed ERP5.CodingStyleTest-TestRunner2

00:41:26

failed ERP5.CodingStyleTest-TestRunner2

05:18:38

failed ERP5.UnitTest-TestRunner2

04:43:58

failed ERP5.UnitTest-TestRunner2

03:34:49

passed ERP5.UnitTest-TestRunner2

08:53:02

failed ERP5.UnitTest-Zope2-ForTestRunner2

03:09:51

failed ERP5.UnitTest-Zope2-ForTestRunner2

02:02:30

failed ERP5.UnitTest-Zope2-ForTestRunner2

08:50:09