Don't impute population=0 or NA with 1

This was breaking the plots for cities like Dublin, CA, and Poway, CA,
which were missing population data from 1980-1989.
The units per capita numbers were nonsensical when imputed with 1.

Author: sid-kap

python/housing_data/build_data_utils.py

@@ -167,7 +167,7 @@ def write_list_json(
 
 def add_per_capita_columns(df: pd.DataFrame, data_sources: list[DataSource]) -> None:
     # There are three cities (Sitka, Weeki Wachee, and Carlton Landing) that had population 0 in some years
-    population = df["population"].where(df["population"] != 0, 1)
+    population = df["population"].replace({0: pd.NA})
 
     cols = {
         col