Added empty string '' to null transform when parsing CSVs

no issue

- When items are parsed from CSV empty values were interpreted as empty strings - ''. Empty strings are always transformed into 'null' values in Ghost's model layer and are much more problematic to validate comparing to plain `null`. Specifically validation was imossible for 'format: date-time' with JSON schema validation through ajv when the value of date property was an empty string
- This behavior resemples one present in Ghost's model layer  - 95880dddeb
- When testing performance overhead for this change did not spot any statistically significant change in performance (tested set was 50K rows)

ghost/members-csv/lib/parse.js

@@ -64,7 +64,14 @@ const readCSV = ({path, columnsToExtract, mapping}) => {
             reject(err);
         })
             .pipe(papaparse.parse(papaparse.NODE_STREAM_INPUT, {
-                header: true
+                header: true,
+                transform: function (value) {
+                    if (value === '') {
+                        return null;
+                    }
+
+                    return value;
+                }
             }))
             .on('data', function (row) {
                 rows.push(row);

ghost/members-csv/test/fixtures/multiple-records-with-empty-values.csv

@@ -0,0 +1,3 @@
+email,name
+"jbloggs@example.com","Bob"
+test@example.com,

ghost/members-csv/test/parse.test.js

@@ -112,4 +112,18 @@ describe('parse', function () {
         result[1].nombre.should.eql('test');
         result[1].id.should.eql('2');
     });
+
+    it('read csv: transforms empty values to nulls', async function () {
+        const result = await readCSV({
+            path: csvPath + 'multiple-records-with-empty-values.csv'
+        });
+
+        should.exist(result);
+        result.length.should.eql(2);
+        result[0].email.should.eql('jbloggs@example.com');
+        result[0].name.should.eql('Bob');
+
+        result[1].email.should.eql('test@example.com');
+        should.equal(result[1].name, null);
+    });
 });