fix: improve error msg on calling json on void function

[ghost]

Closes #278

The error returned on calling json() after execution of a void function is:

Error: DataConversion

Caused by:
    EOF while parsing a value at line 1 column 0

The change proposed gives more context to the error as such:

Error: DataConversion

Caused by:
    the function call returned an empty value, which cannot be parsed as JSON

[ghost]

I added an example though I don't feel it's completely necessary but I'll leave as is and wait for feedback.
I've also added commentary at the top of the implementation why it's an nice add-on.

[ghost]

@frol This PR is ready to be reviewed