Fixed errors when processing element group and merged cells.

[aioutecism]

Fixed the following 2 potential errors:

1

child can be element rather than Array, which will cause error in the inner getElementTexts.

case SlidesApp.PageElementType.GROUP:
  element.asGroup().getChildren().forEach(function(child) {
    texts = texts.concat(getElementTexts(child));
  });
  break;

2

getText can only be called on the HEAD cell of merged cells.

texts.push(table.getCell(x, y).getText());