Resolve "[Bug] Program crashes when no built area is present within a district"

Closes #5 (closed)

\approve @tara @shinde

exposureinitializer/exposureinitializer.py

@@ -225,7 +225,10 @@ class ExposureInitializer:
         if isinstance(self.exposure_db, SpatiaLiteExposure):
             where_clause = "WHERE built_area_size IS NOT NULL"
         else:
-            where_clause = "WHERE A.geometry && B.border AND built_area_size IS NOT NULL"
+            where_clause = """
+                WHERE A.geometry && B.border AND built_area_size IS NOT NULL
+                AND built_area_size > 0
+                """
         sql_statement = f"""
             WITH T AS
             (