Fix crash when painting empty tables (#17952)

packages/flutter/lib/src/rendering/table.dart

@@ -1119,8 +1119,10 @@ class RenderTable extends RenderBox {
   void paint(PaintingContext context, Offset offset) {
     assert(_children.length == rows * columns);
     if (rows * columns == 0) {
-      final Rect borderRect = new Rect.fromLTWH(offset.dx, offset.dy, size.width, 0.0);
-      border.paint(context.canvas, borderRect, rows: const <double>[], columns: const <double>[]);
+      if (border != null) {
+        final Rect borderRect = new Rect.fromLTWH(offset.dx, offset.dy, size.width, 0.0);
+        border.paint(context.canvas, borderRect, rows: const <double>[], columns: const <double>[]);
+      }
       return;
     }
     assert(_rowTops.length == rows + 1);

packages/flutter/test/widgets/table_test.dart

@@ -19,6 +19,14 @@ class TestStatefulWidgetState extends State<TestStatefulWidget> {
 }
 
 void main() {
+  testWidgets('Table widget - empty', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new Table(),
+      ),
+    );
+  });
   testWidgets('Table widget - control test', (WidgetTester tester) async {
     Future<Null> run(TextDirection textDirection) async {
       await tester.pumpWidget(