Update weight funcs, add typing and doc string

saleor/core/weight.py

@@ -35,7 +35,7 @@ def zero_weight():
     return Weight(kg=0)
 
 
-def convert_weight(weight, unit):
+def convert_weight(weight: Weight, unit: str) -> Weight:
     # Weight amount from the Weight instance can be retrived in serveral units
     # via its properties. eg. Weight(lb=10).kg
     converted_weight = getattr(weight, unit)
@@ -49,10 +49,12 @@ def get_default_weight_unit():
     return site.settings.default_weight_unit
 
 
-def covert_weight_to_default_weight_unit(weight):
+def covert_weight_to_default_weight_unit(weight: Weight) -> Weight:
+    """Weight is kept in one unit, but should be returned in site default unit."""
     default_unit = get_default_weight_unit()
-    if weight and weight.unit != default_unit:
-        weight = convert_weight(weight, default_unit)
-    elif weight:
-        weight.value = round(weight.value, 3)
+    if weight is not None:
+        if weight.unit != default_unit:
+            weight = convert_weight(weight, default_unit)
+        else:
+            weight.value = round(weight.value, 3)
     return weight