fix issue #8499 modelChoiceList call getPrimaryKey on a non object
merged branch jaugustin/fix-propel-modelChoiceList (PR #8505)

This PR was merged into the 2.2 branch.

Discussion
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[bridge] [propel1] [ModelChoiceList] fix issue #8499 call getPrimaryKey on a non object

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8499
| License       | MIT
| Doc PR        | no

This fix an issue #8499 introduced by the PR #8223

CC @willdurand @havvg

Commits
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2ebb783 fix issue #8499 modelChoiceList call getPrimaryKey on a non object