Make the weather table upsert non-destructive across sources

ON CONFLICT DO UPDATE was a blind full-row overwrite. Open-data
station rows always carry null visibilityMin/dewPoint/sunDuration
and an empty phenomena array (fields it doesn't publish), so once a
second source (FTP) writes real values for those fields, a later
open-data write for the same (city, dateTime) would silently null
them back out. Switched to COALESCE(excluded.field, weather.field)
so a missing value from one source never erases a real value the
other already wrote; phenomena needs NULLIF against an empty array
specifically, since Scala's List[String] never maps to SQL NULL.

src/main/resources/db/insert_weather_table.sql

@@ -2,17 +2,17 @@ INSERT INTO weather (dateTime, city, tempMax, tempMin, tempAvg, precipitation, w
     VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
     ON CONFLICT (city, dateTime)
     DO UPDATE SET
-        tempMax = excluded.tempMax,
-        tempMin = excluded.tempMin,
-        tempAvg = excluded.tempAvg,
-        precipitation = excluded.precipitation,
-        windAvg = excluded.windAvg,
-        windMax = excluded.windMax,
-        visibilityMin = excluded.visibilityMin,
-        visibilityAvg = excluded.visibilityAvg,
-        snowAvg = excluded.snowAvg,
-        atmPressure = excluded.atmPressure,
-        dewPoint = excluded.dewPoint,
-        humidity = excluded.humidity,
-        sunDuration = excluded.sunDuration,
-        phenomena = excluded.phenomena;
+        tempMax = COALESCE(excluded.tempMax, weather.tempMax),
+        tempMin = COALESCE(excluded.tempMin, weather.tempMin),
+        tempAvg = COALESCE(excluded.tempAvg, weather.tempAvg),
+        precipitation = COALESCE(excluded.precipitation, weather.precipitation),
+        windAvg = COALESCE(excluded.windAvg, weather.windAvg),
+        windMax = COALESCE(excluded.windMax, weather.windMax),
+        visibilityMin = COALESCE(excluded.visibilityMin, weather.visibilityMin),
+        visibilityAvg = COALESCE(excluded.visibilityAvg, weather.visibilityAvg),
+        snowAvg = COALESCE(excluded.snowAvg, weather.snowAvg),
+        atmPressure = COALESCE(excluded.atmPressure, weather.atmPressure),
+        dewPoint = COALESCE(excluded.dewPoint, weather.dewPoint),
+        humidity = COALESCE(excluded.humidity, weather.humidity),
+        sunDuration = COALESCE(excluded.sunDuration, weather.sunDuration),
+        phenomena = COALESCE(NULLIF(excluded.phenomena, ARRAY[]::text[]), weather.phenomena);