[FIX] rma_sale: fix _get_price_unit to handle dropship cases

[AlexPForgeFlow]

Take negative valuation layers, as are to customers, and we handle dropship cases where will have negative and positive layers.
Up to now with dropship orders, sums from valuation layers were 0 and div by 0 error was raised.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.80%. Comparing base (53bb77a) to head (93b827f).
Report is 3 commits behind head on 16.0.

Additional details and impacted files

@@            Coverage Diff             @@
##             16.0     #540      +/-   ##
==========================================
+ Coverage   80.71%   80.80%   +0.08%     
==========================================
  Files         151      159       +8     
  Lines        5295     5387      +92     
  Branches      856      868      +12     
==========================================
+ Hits         4274     4353      +79     
- Misses        783      793      +10     
- Partials      238      241       +3     

Files with missing lines	Coverage Δ
rma_sale/models/rma_order_line.py	72.58% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[AaronHForgeFlow]

LGTM

[LoisRForgeFlow]

LGTM