[Copilot]

StringObject::split() could throw a TypeError, and ArrayObject::avg() returned null for non-empty arrays whose sum was zero.

• String splitting: Route split() through StringObject::explode().
• Array averages: Return null only for empty arrays; zero-sum arrays now return their calculated average.

str('a,b')->split(',')->dump(); // ['a', 'b']
arr([1, -1])->avg();            // 0

• Fixes [Scalars] type bugs #1394