Amount: for total price, acquire price on resource only if unset

When a user explicitely enters 0 on a Trade Model Line, total price must be 0
no matter what is set on the resource.

product/ERP5/Document/Amount.py

@@ -408,7 +408,7 @@ class Amount(Base, VariatedMixin):
       
     """
     price = self.getPrice()
-    if not price:
+    if price is None:
       price = self.getResourcePrice()
     quantity = self.getNetConvertedQuantity()
     if isinstance(price, (int, float)) and isinstance(quantity, (int, float)):

product/ERP5/tests/testComplexTradeModelLineUseCase.py

@@ -373,7 +373,7 @@ return lambda delivery_amount, base_application, **kw: \\
     self.appendBaseContributionCategory(order['1'], poster_present_3cd)
     self.commit()
     self.getAggregatedAmountDict(order, partial_check=True,
-      SPECIAL_DISCOUNT_3CD_OR_1DVD_FIXED=dict(total_price=None),
+      SPECIAL_DISCOUNT_3CD_OR_1DVD_FIXED=dict(total_price=0),
       TOTAL_PRICE_WITHOUT_VAT=dict(total_price=6000),
       TOTAL_PRICE_WITH_VAT=dict(total_price=6300),
       VAT_AMOUNT=dict(total_price=300))
@@ -386,7 +386,7 @@ return lambda delivery_amount, base_application, **kw: \\
     self.appendBaseContributionCategory(line, poster_present_1dvd)
     self.commit()
     self.getAggregatedAmountDict(order, partial_check=True,
-      SPECIAL_DISCOUNT_3CD_OR_1DVD_FIXED=dict(total_price=None, quantity=1,
+      SPECIAL_DISCOUNT_3CD_OR_1DVD_FIXED=dict(total_price=0, quantity=1,
                                               resource_value=self.poster),
       TOTAL_PRICE_WITHOUT_VAT=dict(total_price=9000),
       TOTAL_PRICE_WITH_VAT=dict(total_price=9450),
@@ -400,7 +400,7 @@ return lambda delivery_amount, base_application, **kw: \\
     self.appendBaseContributionCategory(line, poster_present_3cd)
     self.commit()
     self.getAggregatedAmountDict(order, partial_check=True,
-      SPECIAL_DISCOUNT_3CD_OR_1DVD_FIXED=dict(total_price=None, quantity=1,
+      SPECIAL_DISCOUNT_3CD_OR_1DVD_FIXED=dict(total_price=0, quantity=1,
                                               resource_value=self.poster),
       TOTAL_PRICE_WITHOUT_VAT=dict(total_price=11400),
       TOTAL_PRICE_WITH_VAT=dict(total_price=11970),