[web] Fix layout exception when text is null

lib/web_ui/lib/src/engine/text/layout_service.dart

@@ -71,6 +71,10 @@ class TextLayoutService {
     didExceedMaxLines = false;
     lines.clear();
 
+    if (spanCount == 0) {
+      return;
+    }
+
     final Spanometer spanometer = Spanometer(paragraph, context);
 
     int spanIndex = 0;

lib/web_ui/test/text/layout_service_plain_test.dart

@@ -42,6 +42,16 @@ void main() {
 void testMain() async {
   await ui.webOnlyInitializeTestDomRenderer();
 
+  test('no text', () {
+    final CanvasParagraph paragraph = CanvasParagraphBuilder(ahemStyle).build();
+    paragraph.layout(constrain(double.infinity));
+
+    expect(paragraph.maxIntrinsicWidth, 0);
+    expect(paragraph.minIntrinsicWidth, 0);
+    expect(paragraph.height, 0);
+    expect(paragraph.computeLineMetrics(), isEmpty);
+  });
+
   test('preserves whitespace when measuring', () {
     CanvasParagraph paragraph;