[Locked Figure Labels] Fix bug that's causing the editor to crash whe…

…n the label input is empty (and on load) (#1856)

## Summary:
I need to add a null check after my $ TeX updates in #1847.

This is actually already added in a larger PR, but this fix needs to get out ASAP.

Issue: none

## Test plan:
- Go to http://localhost:6006/iframe.html?id=perseuseditor-widgets-interactive-graph--mafs-with-locked-figure-labels-all-flags&viewMode=story
- Backspace in the locked point visible label input field
- Confirm that there is no crash

Author: nishasy

Reviewers: benchristel, catandthemachines, anakaren-rojas

Required Reviewers:

Approved By: benchristel

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

Pull Request URL: #1856

.changeset/dry-moles-notice.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus": patch
+---
+
+[Locked Figure Labels] Fix bug that's causing the editor to crash when the label input is empty (and on load)

packages/perseus/src/widgets/interactive-graphs/utils.ts

@@ -109,9 +109,13 @@ type ParsedNode = {
 
 // Helper function for replaceOutsideTeX()
 // Condense adjacent text nodes into a single text node
-function condenseTextNodes(nodes: Array<ParsedNode>): Array<ParsedNode> {
+function condenseTextNodes(nodes: ParsedNode[] | undefined): Array<ParsedNode> {
     const result: ParsedNode[] = [];
 
+    if (!nodes) {
+        return result;
+    }
+
     let currentText = "";
     for (const node of nodes) {
         if (node.type === "math") {