Added migration to normalize all empty strings to NULL (#11132)

refs https://github.com/TryGhost/Ghost/issues/10388, original PR https://github.com/TryGhost/Ghost/pull/10428

- re-introduces the migration which normalizes all empty strings in the database to `NULL`
- this was previously reverted due to being too large of a migration for a minor but a major is a good time to do it

core/server/data/migrations/versions/3.0/10-remove-empty-strings.js

@@ -0,0 +1,81 @@
+const Promise = require('bluebird');
+const common = require('../../../../lib/common');
+const schema = require('../../../schema');
+
+/*
+ * [{
+ *   tableName: 'posts',
+ *   columns: ['custom_excerpt', 'description', 'etc...']
+ * }]
+ * */
+const tablesToUpdate = Object.keys(schema.tables).reduce((tablesToUpdate, tableName) => {
+    const table = schema.tables[tableName];
+    const columns = Object.keys(table).filter((columnName) => {
+        const column = table[columnName];
+        return column.nullable && ['string', 'text'].includes(column.type);
+    });
+    if (!columns.length) {
+        return tablesToUpdate;
+    }
+    return tablesToUpdate.concat({
+        tableName,
+        columns
+    });
+}, []);
+
+const createReplace = (connection, from, to) => (tableName, columnName) => {
+    return connection.schema.hasTable(tableName)
+        .then((tableExists) => {
+            if (!tableExists) {
+                common.logging.warn(
+                    `Table ${tableName} does not exist`
+                );
+                return;
+            }
+            return connection.schema.hasColumn(tableName, columnName)
+                .then((columnExists) => {
+                    if (!columnExists) {
+                        common.logging.warn(
+                            `Table '${tableName}' does not have column '${columnName}'`
+                        );
+                        return;
+                    }
+
+                    common.logging.info(
+                        `Updating ${tableName}, setting '${from}' in ${columnName} to '${to}'`
+                    );
+
+                    return connection(tableName)
+                        .where(columnName, from)
+                        .update(columnName, to);
+                });
+        });
+};
+
+module.exports.up = ({transacting}) => {
+    const replaceEmptyStringWithNull = createReplace(transacting, '', null);
+
+    return Promise.all(
+        tablesToUpdate.map(({tableName, columns}) => Promise.all(
+            columns.map(
+                columnName => replaceEmptyStringWithNull(tableName, columnName)
+            )
+        ))
+    );
+};
+
+module.exports.down = ({connection}) => {
+    const replaceNullWithEmptyString = createReplace(connection, null, '');
+
+    return Promise.all(
+        tablesToUpdate.map(({tableName, columns}) => Promise.all(
+            columns.map(
+                columnName => replaceNullWithEmptyString(tableName, columnName)
+            )
+        ))
+    );
+};
+
+module.exports.config = {
+    transaction: true
+};