const style = TextStyle(height: 11, fontSize: 7);
const simple = TextSpan(text: 'x', style: style);
const emptyString = TextSpan(text: '', style: style);
const emptyParagraph = TextSpan(style: style);

final painter = TextPainter(
  textDirection: TextDirection.ltr,
  strutStyle: StrutStyle.fromTextStyle(style),
  textHeightBehavior: const TextHeightBehavior(applyHeightToFirstAscent: false, applyHeightToLastDescent: false),
);

for (final span in [simple, emptyString, emptyParagraph]) {
  painter.text = span;
  painter.layout();
  final realHeight = painter.height;
  final preferredHeight = painter.preferredLineHeight;
  print('${span.text} > real height: $realHeight, preferred height: $preferredHeight');
}