Only set promo configuration to legacy if no other set

[mamhoff]

This sets the promotion configuration to the legacy promotion system if no other promotion system, like solidus_promotion has been configured in the spree initializer.

This should get rid of errors people encounter when trying to use solidus_promotions, but the system being reconfigured without their intervention to use the legacy system.

Here's the problem this should solve:

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.54%. Comparing base (8ff6267) to head (f4d24f2). Report is 8 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5928 +/- ## ========================================== + Coverage 87.81% 89.54% +1.72% ========================================== Files 476 782 +306 Lines 11656 17988 +6332 ========================================== + Hits 10236 16107 +5871 - Misses 1420 1881 +461 ```

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