Added failing test for #1898.

Plus a couple of other empty binding path tests.

tests/Avalonia.Markup.UnitTests/Data/BindingTests.cs

@@ -273,6 +273,39 @@ namespace Avalonia.Markup.UnitTests.Data
             Assert.Equal(42, target.Value);
         }
 
+        [Fact]
+        public void Null_Path_Should_Bind_To_DataContext()
+        {
+            var target = new TextBlock { DataContext = "foo" };
+            var binding = new Binding();
+
+            target.Bind(TextBlock.TextProperty, binding);
+
+            Assert.Equal("foo", target.Text);
+        }
+
+        [Fact]
+        public void Empty_Path_Should_Bind_To_DataContext()
+        {
+            var target = new TextBlock { DataContext = "foo" };
+            var binding = new Binding { Path = string.Empty };
+
+            target.Bind(TextBlock.TextProperty, binding);
+
+            Assert.Equal("foo", target.Text);
+        }
+
+        [Fact]
+        public void Dot_Path_Should_Bind_To_DataContext()
+        {
+            var target = new TextBlock { DataContext = "foo" };
+            var binding = new Binding { Path = "." };
+
+            target.Bind(TextBlock.TextProperty, binding);
+
+            Assert.Equal("foo", target.Text);
+        }
+
         /// <summary>
         /// Tests a problem discovered with ListBox with selection.
         /// </summary>

tests/Avalonia.Markup.UnitTests/Parsers/ExpressionNodeBuilderTests.cs

@@ -36,6 +36,15 @@ namespace Avalonia.Markup.UnitTests.Parsers
             AssertIsProperty(result[0], "F0o");
         }
 
+        [Fact]
+        public void Should_Build_Dot()
+        {
+            var result = ToList(ExpressionObserverBuilder.Parse("."));
+
+            Assert.Equal(1, result.Count);
+            Assert.IsType<EmptyExpressionNode>(result[0]);
+        }
+
         [Fact]
         public void Should_Build_Property_Chain()
         {

tests/Avalonia.Markup.UnitTests/Parsers/ExpressionNodeBuilderTests_Errors.cs

@@ -30,6 +30,13 @@ namespace Avalonia.Markup.UnitTests.Parsers
                 () => ExpressionObserverBuilder.Parse("Foo.Bar."));
         }
 
+        [Fact]
+        public void Expression_Cannot_Start_With_Period_Then_Token()
+        {
+            Assert.Throws<ExpressionParseException>(
+                () => ExpressionObserverBuilder.Parse(".Bar"));
+        }
+
         [Fact]
         public void Expression_Cannot_Have_Empty_Indexer()
         {