Better message when clicking Run before writing any code

**Program:** https://code.world/#P1B2M2Y8AsgTpgAmY7PhCfg

**Message:**

program.hs:1:1: error:
The variable program is not defined in your code

Reproduce the empty-program case using the linked CodeWorld program and confirm the current program.hs:1:1 error. Then trace the Run handling and compiler-error path; done means clicking Run with no code shows a clear empty-program message instead of reporting that program is undefined.

Scritto dal modello di indicizzazione a partire dal testo della issue.