[15.0][FIX] purchase_representative: allow to get the representative in MRP

[alan196]

This PR fixes the following issue:

Steps to reproduce:

• Configure raw material as MTO.
• Create a mrp.production with that raw material.

Current behavior:

• The PO is created with representative Odoobot.

Expected behavior:

• The PO is created with the user that confirms the production

[github-actions[bot]]

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