🪲 Add support for empty strings to convert_numerals prefix function

[boryanagoncharenko]

Fixes #5341
The prefix function convert_numerals now has support for empty strings and None values.

How to test
Run Hedy locally and navigate to level 12 and run:

for i in range 1 to 3
    form = ask "What form?"
    color = ask "What color?"
    if form is 'test'
        print 'test'

Once the program is running do the following:

1. Supply any value to the first input, e.g. a
2. Supply any color to the second input, e.g. red
3. Cancel the program by clicking the 'Stop program' red button
4. Start the program again
5. Supply any value to the first input, e.g. b
6. Supply any color to the second input, e.g. red
7. Check that there is no 'string index out of range' error

[jpelay]

It works!!! It also complements nicely with the fix we did in #5225 (which is te reason why stoppped programs keep reappearing) Ultimately, I want to solve the problem for good, with stopped programs being actually stop, but I'm not sure how to tackle that with the current Skulpt implementation, I would love your help on that!

[mergify]

Thank you for contributing! Your pull request is now going on the merge train (choo choo! Do not click update from main anymore, and be sure to allow changes to be pushed to your fork).

[mergify]

Thank you for contributing! Your pull request is now going on the merge train (choo choo! Do not click update from main anymore, and be sure to allow changes to be pushed to your fork).