Add validation for empty user input

packages/perseus/src/widgets/matrix/validate-matrix.test.ts

@@ -3,7 +3,33 @@ import validateMatrix from "./validate-matrix";
 import type {PerseusMatrixUserInput} from "../../validation.types";
 
 describe("matrixValidator", () => {
-    it("is null always", () => {
+    it("should return invalid when answers is completely empty", () => {
+        // Arrange
+        const userInput: PerseusMatrixUserInput = {
+            answers: [],
+        };
+
+        // Act
+        const result = validateMatrix(userInput, {});
+
+        // Assert
+        expect(result).toHaveInvalidInput();
+    });
+
+    it("should return invalid when any answer row is empty", () => {
+        // Arrange
+        const userInput: PerseusMatrixUserInput = {
+            answers: [[1, 2, 3], [], [7, 8, 9]],
+        };
+
+        // Act
+        const result = validateMatrix(userInput, {});
+
+        // Assert
+        expect(result).toHaveInvalidInput();
+    });
+
+    it("should return null for non-empty user input", () => {
         // Arrange
         const userInput: PerseusMatrixUserInput = {
             answers: [

packages/perseus/src/widgets/matrix/validate-matrix.ts

@@ -7,7 +7,7 @@ import type {
 /**
  * Checks user input from the matrix widget to see if it is scorable.
  *
- * Note: The matrix widget cannot do any validation without the Scoring
+ * Note: The matrix widget cannot do much validation without the Scoring
  * Data because of its use of KhanAnswerTypes as a core part of scoring.
  *
  * @see `scoreMatrix()` for more details.
@@ -16,6 +16,14 @@ function validateMatrix(
     userInput: PerseusMatrixUserInput,
     rubric: PerseusMatrixValidationData,
 ): Extract<PerseusScore, {type: "invalid"}> | null {
+    // Very basic check: did we get 0 rows or any rows with 0 answers? If so,
+    // then the user input is not gradable.
+    if (
+        userInput.answers.length === 0 ||
+        userInput.answers.some((row) => row.length === 0)
+    ) {
+        return {type: "invalid", message: null};
+    }
     return null;
 }