Add test to document empty expression can be a correct answer (#2003)

## Summary:
RendererUtil: remove unnecessary cast to UserInput

Issue: LEMS-2561


## Test plan:

Author: jeremywiebe

Reviewers: jeremywiebe, handeyeco, Myranae

Required Reviewers:

Approved By: handeyeco

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

Pull Request URL: #2003

packages/perseus/src/widgets/numeric-input/score-numeric-input.test.ts

@@ -6,13 +6,39 @@ import scoreNumericInput, {maybeParsePercentInput} from "./score-numeric-input";
 
 import type {PerseusNumericInputRubric} from "../../validation.types";
 
-describe("static function validate", () => {
+describe("scoreNumericInput", () => {
     beforeEach(() => {
         jest.spyOn(Dependencies, "getDependencies").mockReturnValue(
             testDependencies,
         );
     });
 
+    it("is correct when input is empty but answer is 1 and coefficient: true", () => {
+        const rubric: PerseusNumericInputRubric = {
+            answers: [
+                {
+                    value: 1,
+                    status: "correct",
+                    maxError: 0,
+                    simplify: "optional",
+                    strict: false,
+                    message: "",
+                },
+            ],
+            coefficient: true,
+        };
+
+        const userInput = {
+            // Empty input being translated to "1" depends on coefficient being
+            // true.
+            currentValue: "",
+        };
+
+        const score = scoreNumericInput(userInput, rubric, mockStrings);
+
+        expect(score).toHaveBeenAnsweredCorrectly();
+    });
+
     it("with a simple value", () => {
         const rubric: PerseusNumericInputRubric = {
             answers: [