make data_warning empty instead of null

Makefile

@@ -96,7 +96,7 @@ do-import-spreadsheets:
 	echo 'ALTER TABLE "referendums" ADD COLUMN id SERIAL PRIMARY KEY;' | psql $(DATABASE_NAME)
 	./bin/remove-whitespace $(DATABASE_NAME) referendums Short_Title
 	./bin/remove-whitespace $(DATABASE_NAME) referendums Summary
-	./bin/remove-whitespace $(DATABASE_NAME) referendums data_warning
+	./bin/make-null-empty $(DATABASE_NAME) referendums data_warning
 
 	echo 'DROP TABLE IF EXISTS name_to_number CASCADE;' | psql $(DATABASE_NAME)
 	./bin/create-table $(DATABASE_NAME) $(CSV_PATH) name_to_number

bin/make-null-empty

@@ -0,0 +1,20 @@
+#!/bin/bash
+# Usage: ./bin/make-null-empty [database name] [table name] [column name]
+#        ./bin/make-null-empty disclosure-backend candidates Candidate
+set -euo pipefail
+
+if [ $# -ne 3 ]; then
+  echo 'Usage: ./bin/make-null-empty [database name] [table name] [column name]'
+  exit 1
+fi
+
+database_name=$1
+table_name=$2
+column_name=$3
+
+# trim leading and trailing white spaces
+cat <<-QUERY | psql ${database_name}
+  \\set ON_ERROR_STOP on
+  UPDATE "$table_name" t SET "$column_name" = '' WHERE "$column_name" IS NULL;
+
+QUERY