Fix empty meta fields input in Advanced Settings (elastic#78576)

Author: kertal

src/plugins/advanced_settings/public/management_app/components/form/__snapshots__/form.test.tsx.snap

@@ -91,6 +91,16 @@ const settings = {
       description: 'foo',
       category: ['general'],
     },
+    {
+      ...defaults,
+      name: 'general:test:array',
+      ariaName: 'array test',
+      displayName: 'Test array setting',
+      description: 'array foo',
+      type: 'array' as UiSettingsType,
+      category: ['general'],
+      defVal: ['test'],
+    },
   ],
   'x-pack': [
     {
@@ -256,4 +266,60 @@ describe('Form', () => {
       })
     );
   });
+
+  it('should save an array typed field when user provides an empty string correctly', async () => {
+    const wrapper = mountWithI18nProvider(
+      <Form
+        settings={settings}
+        visibleSettings={settings}
+        categories={categories}
+        categoryCounts={categoryCounts}
+        save={save}
+        clearQuery={clearQuery}
+        showNoResultsMessage={true}
+        enableSaving={false}
+        toasts={{} as any}
+        dockLinks={{} as any}
+      />
+    );
+
+    (wrapper.instance() as Form).setState({
+      unsavedChanges: {
+        'general:test:array': {
+          value: '',
+        },
+      },
+    });
+
+    findTestSubject(wrapper.update(), `advancedSetting-saveButton`).simulate('click');
+    expect(save).toHaveBeenCalledWith({ 'general:test:array': [] });
+  });
+
+  it('should save an array typed field when user provides a comma separated string correctly', async () => {
+    const wrapper = mountWithI18nProvider(
+      <Form
+        settings={settings}
+        visibleSettings={settings}
+        categories={categories}
+        categoryCounts={categoryCounts}
+        save={save}
+        clearQuery={clearQuery}
+        showNoResultsMessage={true}
+        enableSaving={false}
+        toasts={{} as any}
+        dockLinks={{} as any}
+      />
+    );
+
+    (wrapper.instance() as Form).setState({
+      unsavedChanges: {
+        'general:test:array': {
+          value: 'test1, test2',
+        },
+      },
+    });
+
+    findTestSubject(wrapper.update(), `advancedSetting-saveButton`).simulate('click');
+    expect(save).toHaveBeenCalledWith({ 'general:test:array': ['test1', 'test2'] });
+  });
 });

src/plugins/advanced_settings/public/management_app/components/form/form.test.tsx

@@ -154,7 +154,9 @@ export class Form extends PureComponent<FormProps> {
       let equalsToDefault = false;
       switch (type) {
         case 'array':
-          valueToSave = valueToSave.split(',').map((val: string) => val.trim());
+          valueToSave = valueToSave.trim();
+          valueToSave =
+            valueToSave === '' ? [] : valueToSave.split(',').map((val: string) => val.trim());
           equalsToDefault = valueToSave.join(',') === (defVal as string[]).join(',');
           break;
         case 'json':