SVG <circle> without cx/cy attributes crashes in SvgPath.fromCircleXml

[tornadoblue]

Describe the bug

The SVG spec defines cx and cy as optional attributes on <circle>, defaulting to 0 (SVG spec ref). However, SvgPath.fromCircleXml in pdf/src/svg/path.dart:105 uses null assertions (!) on the return value of SvgParser.getNumeric() for these attributes, causing a crash when they're omitted.

To Reproduce

Code snippet to reproduce the behavior:

import 'package:pdf/pdf.dart';
import 'package:pdf/widgets.dart' as pw;

void main() async {
  final pdf = pw.Document();

  // Valid SVG per spec — cx and cy are optional, default to 0
  const svg = '<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">'
      '<circle r="20" fill="red"/>'
      '</svg>';

  pdf.addPage(
    pw.Page(
      build: (context) => pw.SvgImage(svg: svg, width: 100, height: 100),
    ),
  );

  await pdf.save(); // Crashes here during paint phase
}

Expected behavior

The <circle r="20"/> element should render as a circle at (0, 0) with radius 20, matching browser SVG rendering behavior. Instead, pdf.save() throws Null check operator used on a null value.

Suggested fix in pdf/src/svg/path.dart:

// Current (crashes):
final cx = SvgParser.getNumeric(element, 'cx', _brush)!.sizeValue;
final cy = SvgParser.getNumeric(element, 'cy', _brush)!.sizeValue;

// Fixed (defaults to 0 per SVG spec):
final cx = SvgParser.getNumeric(element, 'cx', _brush)?.sizeValue ?? 0;
final cy = SvgParser.getNumeric(element, 'cy', _brush)?.sizeValue ?? 0;

Screenshots

N/A — server-side PDF generation, no UI.

Flutter Doctor

N/A — Dart-only server environment (Serverpod v3.3), not a Flutter app.
pdf package version: 3.11.3
Dart SDK: 3.5

Desktop (please complete the following information):

• Linux

Additional context

This affects real-world SVG flag files (e.g. from flag-icons) where <circle> elements commonly omit cx/cy when centered at the origin. Examples: India's flag (<circle r="20" fill="#008"/>), Uruguay, Rwanda.

Our workaround is pre-processing SVGs to inject the missing attributes before passing them to pw.SvgImage.

[tmbenhura]

The correct implementation would be:

final cx = SvgParser.getNumeric(element, 'cx', _brush, defaultValue: 0).sizeValue;
final cy = SvgParser.getNumeric(element, 'cy', _brush, defaultValue: 0).sizeValue;