Fix evaluation error when no evaluation code is provided (#1472)

* Fix invalid array access for stepper steps

* Resolves bug where no execution steps will lead to access error

* Add evaluation message to indicate no evaluation

* Returns empty step with message

* Test evaluation of empty programs in stepper

---------

Co-authored-by: Martin Henz <[email protected]>

Author: sayomaki

src/stepper/__tests__/__snapshots__/stepper.ts.snap

@@ -2095,6 +2095,8 @@ false ? 1 : 100 * factorial(100 - 1);
 "
 `;
 
+exports[`Evaluation of empty code and imports Evaluate empty program 1`] = `""`;
+
 exports[`Infinite recursion 1`] = `
 "function f() {
   return f();

src/stepper/__tests__/stepper.ts

@@ -1433,6 +1433,16 @@ describe(`#1342: Test the fix of #1341: Stepper limit off by one`, () => {
   })
 })
 
+describe(`Evaluation of empty code and imports`, () => {
+  test('Evaluate empty program', () => {
+    const code = ``
+    const program = parse(code, mockContext())!
+    const steps = getEvaluationSteps(program, mockContext(), 1000)
+    expect(steps.map(x => codify(x[0])).join('\n')).toMatchSnapshot()
+    expect(getLastStepAsString(steps)).toEqual('')
+  })
+})
+
 // describe(`#1223: Stepper: Import statements cause errors`, () => {
 //   test('import a module and invoke its functions', () => {
 //     const code = `

src/stepper/stepper.ts

@@ -3383,9 +3383,12 @@ export function getEvaluationSteps(
       reducedWithPath = reduceMain(reducedWithPath[0], context)
       i += 2
     }
-    if (!limitExceeded) {
+    if (!limitExceeded && steps.length > 0) {
       steps[steps.length - 1][2] = 'Evaluation complete'
     }
+    if (steps.length === 0) {
+      steps.push([reducedWithPath[0] as es.Program, [], 'Nothing to evaluate'])
+    }
     return steps
   } catch (error) {
     context.errors.push(error)