Add nullability annotations in generated code

[ris58h]

At least a return value of Factory#createElement is always non-null.

// ... enclosing class ...
class Factory {
  public static PsiElement createElement(ASTNode node) {
    IElementType type = node.getElementType();
    // if (type == ...
    throw new AssertionError("Unknown element type: " + type);
  }
}
// ... enclosing class ...