Fix evaluation error when no evaluation code is provided

[sayomaki]

Description

This PR aims to resolve a bug where if there is no code to be evaluated in the playground stepper, an error will be thrown in the page (see attached image below:)

The bug can be replicated by simply executing empty code in the playground stepper, or when modules are imported but no code has been provided, e.g.

import { heart } from "rune";

The first commit attempts to resolve the problem, but no useful information is provided back to the user (looks as if nothing has occurred) (maybe this could be detected and implemented via the frontend?), and thus the second commit will return an evaluation response with a message to signal that nothing has been evaluated (see images below):

New behaviour when nothing is evaluated (click to expand)

If the second commit/user interaction feedback is not required (e.g. implemented in frontend), feel free to let me know and I will remove the commit from this PR.

Testing

These changes have been tested locally, and no longer produces the error as shown above in the first image.

[RichDom2185]

LGTM, thanks a lot! Prof @martin-henz, shall we bump js-slang and quickly deploy the new changes after this?

[coveralls]

Pull Request Test Coverage Report for Build 5999207364

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 82.679%

---

Totals
Change from base Build 5995455758:	0.005%
Covered Lines:	10770
Relevant Lines:	12587

---

💛 - Coveralls

[RichDom2185]

One minor comment, would it be possible to add test cases to ensure this behaviour?

[sayomaki]

Just pushed a simple test for the execution of an empty program (running the test prior to changes from this PR will fail to pass/produce error).

However, it seems that mocking modules have not been fully implemented, so the test for that is not included.