Ignore zero wei transfers (#405)

Today's dry-run failed because of an assertion that fails with the
following msg:

`AssertionError: Can't construct negative transfer of 0`

It seems the script is now trying to add some zero wei transfers for
partner fees. I admit i don't know why these transfers show up now and
weren't showing up before, but for now this PR proposes to relax the
test and only complain for negative transfers for partner fees, while
ignoring the zero wei transfers.

src/fetch/payouts.py

@@ -332,13 +332,16 @@ def prepare_transfers(
             )
         )
     for address in partner_fees_wei:
-        transfers.append(
-            Transfer(
-                token=None,
-                recipient=Address(address),
-                amount_wei=partner_fees_wei[address],
+        amount_wei = partner_fees_wei[address]
+        assert amount_wei >= 0, f"Can't construct negative transfer of {amount_wei}"
+        if amount_wei > 0:
+            transfers.append(
+                Transfer(
+                    token=None,
+                    recipient=Address(address),
+                    amount_wei=amount_wei,
+                )
             )
-        )
 
     return PeriodPayouts(overdrafts, transfers)
 

src/models/transfer.py

@@ -52,7 +52,7 @@ class Transfer:
     amount_wei: int
 
     def __init__(self, token: Optional[Token], recipient: Address, amount_wei: int):
-        assert amount_wei > 0, f"Can't construct negative transfer of {amount_wei}"
+        assert amount_wei > 0, f"Can't construct non-positive transfer of {amount_wei}"
 
         self.token = token
         self._recipient = recipient