BUG: Fix early exit from fixed slippage when a order amount is zero.

The order loop should continue, effectively dropping/skipping the zero
amount order, not return early and erroneously skip possible valid
orders.

zipline/finance/slippage.py

@@ -224,7 +224,7 @@ class FixedSlippage(SlippageModel):
                 continue
 
             if zp_math.tolerant_equals(order.amount, 0):
-                return txns
+                continue
 
             txn = create_transaction(
                 event,