Support MilkProduction activities with only quantity_produced - see HEA-887

Some BSSs (e.g.BF04~2011) have MilkProduction activities with a
quantity_produced but no quantity_sold, and hence no income.
They also have no percentage_kcals because that is initially part
of the ButterProduction activities. This was causing the MilkProduction
to be ignored, and therefore not available when attempting to transfer
the kcals from the ButterProduction to the MilkProduction.  This commit
ensures that strategies that have a `quantity_produced` are kept, even
if they don't have income, expenditure or kcals.

Author: rhunwicks

pipelines/assets/livelihood_activity.py

@@ -771,7 +771,13 @@ def get_instances_from_dataframe(
                             field in livelihood_activity
                             and (livelihood_activity[field] or livelihood_activity[field] == 0)
                         )
-                        for field in ["income", "expenditure", "kcals_consumed", "percentage_kcals"]
+                        for field in [
+                            "quantity_produced",  # MilkProduction may have quantity_produced but not any of the others.
+                            "income",
+                            "expenditure",
+                            "kcals_consumed",
+                            "percentage_kcals",
+                        ]
                     )
                 ]