(core) Use correct empty value in ChoiceEditor.getCellValue

Summary:
The default value of Choice columns is empty string, but ChoiceEditor was
saving nulls whenever a blank value was saved. This was causing unexpected
updates to trigger values due to the cell value changing internally, even
though null and empty string appear the same in the UI.

Test Plan: Browser test.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D4242

app/client/widgets/ChoiceEditor.js

@@ -77,7 +77,7 @@ ChoiceEditor.prototype.getCellValue = function() {
   if (selectedItem) {
     return selectedItem.label;
   } else if (this.textInput.value.trim() === '') {
-    return null;
+    return '';
   } else {
     return TextEditor.prototype.getCellValue.call(this);
   }

test/nbrowser/Choice.ts

@@ -0,0 +1,54 @@
+import {DocAPI, UserAPI} from 'app/common/UserAPI';
+import {assert, Key} from 'mocha-webdriver';
+import * as gu from 'test/nbrowser/gristUtils';
+import {setupTestSuite} from 'test/nbrowser/testUtils';
+
+describe('Choice', function() {
+  this.timeout(20000);
+  const cleanup = setupTestSuite();
+
+  let docId: string;
+  let api: UserAPI;
+  let docApi: DocAPI;
+
+  before(async () => {
+    const session = await gu.session().teamSite.login();
+    docId = await session.tempNewDoc(cleanup, 'Choice');
+    api = session.createHomeApi();
+    docApi = api.getDocAPI(docId);
+  });
+
+  afterEach(() => gu.checkForErrors());
+
+  it('should set cell value to empty string when editor text is blank', async () => {
+    // Add a few records to Table1.
+    await api.applyUserActions(docId, [
+      ['BulkAddRecord', 'Table1', [null, null, null], {}],
+    ]);
+
+    // Change column A's type to Choice and check its values default to empty string.
+    await api.applyUserActions(docId, [
+      ['ModifyColumn', 'Table1', 'A', {
+        type: 'Choice',
+      }],
+    ]);
+    assert.deepEqual(await docApi.getRecords('Table1'), [
+      {id: 1, fields: {A: '', C: null, B: null}},
+      {id: 2, fields: {A: '', C: null, B: null}},
+      {id: 3, fields: {A: '', C: null, B: null}},
+    ]);
+
+    // Start editing a cell in column A and click away to close the editor.
+    await gu.getCell({rowNum: 1, col: 'A'}).click();
+    await gu.sendKeys(Key.ENTER);
+    await gu.getCell({rowNum: 1, col: 'C'}).click();
+    await gu.waitForServer();
+
+    // Check that the values in column A are unchanged.
+    assert.deepEqual(await docApi.getRecords('Table1'), [
+      {id: 1, fields: {A: '', C: null, B: null}},
+      {id: 2, fields: {A: '', C: null, B: null}},
+      {id: 3, fields: {A: '', C: null, B: null}},
+    ]);
+  });
+});