SSS: Remove duplicate nullness check on answerableCallback (#1947)

## Summary:

A super small refactoring of the `componentDidUpdate` to reduce some duplication.

Issue: LEMS-2561

## Test plan:

`yarn test`
`yarn typecheck`

Author: jeremywiebe

Reviewers: handeyeco, Myranae

Required Reviewers:

Approved By: handeyeco

Checks: ✅ gerald, ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x), ✅ gerald

Pull Request URL: #1947

.changeset/mean-fans-boil.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus": patch
+---
+
+Minor refactoring of ServerItemRenderer's componentDidUpdate to reduce duplication

packages/perseus/src/server-item-renderer.tsx

@@ -137,13 +137,11 @@ export class ServerItemRenderer
     }
 
     componentDidUpdate(prevProps: Props, prevState: State) {
-        if (this.props.apiOptions.answerableCallback) {
+        const answerableCallback = this.props.apiOptions.answerableCallback;
+        if (answerableCallback != null) {
             const isAnswerable =
                 this.questionRenderer.emptyWidgets().length === 0;
-            const {answerableCallback} = this.props.apiOptions;
-            if (answerableCallback) {
-                answerableCallback(isAnswerable);
-            }
+            answerableCallback(isAnswerable);
         }
 
         if (!this._fullyRendered) {