[UX] Fix SchemaAddFieldModal stuttering on first new schema field add

- With the new template, transitioning from the empty state to the filled schema state causes the modal to stutter due to the component rerender

- Changing the page to not instantly react/update `hasSchema` when local schema state changes but instead to wait for the server call to finish and for cachedSchema to update fixes the UX problem

x-pack/plugins/enterprise_search/public/applications/app_search/components/schema/schema_logic.test.ts

@@ -140,13 +140,13 @@ describe('SchemaLogic', () => {
 
   describe('selectors', () => {
     describe('hasSchema', () => {
-      it('returns true when the schema obj has items', () => {
-        mountAndSetSchema({ schema: { test: SchemaType.Text } });
+      it('returns true when the cached server schema obj has items', () => {
+        mount({ cachedSchema: { test: SchemaType.Text } });
         expect(SchemaLogic.values.hasSchema).toEqual(true);
       });
 
-      it('returns false when the schema obj is empty', () => {
-        mountAndSetSchema({ schema: {} });
+      it('returns false when the cached server schema obj is empty', () => {
+        mount({ schema: {} });
         expect(SchemaLogic.values.hasSchema).toEqual(false);
       });
     });

x-pack/plugins/enterprise_search/public/applications/app_search/components/schema/schema_logic.ts

@@ -108,7 +108,10 @@ export const SchemaLogic = kea<MakeLogicType<SchemaValues, SchemaActions>>({
     ],
   },
   selectors: {
-    hasSchema: [(selectors) => [selectors.schema], (schema) => Object.keys(schema).length > 0],
+    hasSchema: [
+      (selectors) => [selectors.cachedSchema],
+      (cachedSchema) => Object.keys(cachedSchema).length > 0,
+    ],
     hasSchemaChanged: [
       (selectors) => [selectors.schema, selectors.cachedSchema],
       (schema, cachedSchema) => !isEqual(schema, cachedSchema),