fix: correctly handle empty INCLUDEs

[floriancargoet]

Checklist

• The new code additions passed the tests (npm test).
• The linter ran and found no issues (npm run-script lint).
  • Quelques warnings mais ils étaient là avant.

Description

This PR fixes a bug in the compiler with empty INCLUDEs.

Example:

// main.ink
INCLUDE a.ink
INCLUDE b.ink
{foo}

// a.ink (empty file)

// b.ink
VAR foo = "bar"

If you compile it with the inkjs.Compiler and a correctly set-up JsonFileHandler, the story fails at run-time with Unresolved variable: foo.
If a.ink is not empty (a single space character is enough), everything works as expected.

This bug is caused by a falsy value ("") not taking the correct branch in an if statement if (includedString). This PR fixes that if with a stricter check: if (includedString != null).