Fix busted tests due to a bug in PeriodicValueConverter

Some places was using just .Periodic() without any labels specified which triggered the debug assertion that the label count was 2 or 4. Because all the value converter stuff is static, the exception raised during the static constructor crashed the XUnit test runner process and masked the fact these tests were failing because they was never actually being run.

Fixed by treating the case of no labels the same as 4 for backwards compatibility.

Author: lee-m

src/ExCSS/ValueConverters/PeriodicValueConverter.cs

@@ -11,7 +11,7 @@ internal sealed class PeriodicValueConverter : IValueConverter
 
         public PeriodicValueConverter(IValueConverter converter, string[] labels)
         {
-            Debug.Assert(labels.Length == 4 || labels.Length == 2);
+            Debug.Assert(labels.Length == 0 || labels.Length == 4 || labels.Length == 2);
 
             _converter = converter;
             _labels = labels;
@@ -20,7 +20,7 @@ public PeriodicValueConverter(IValueConverter converter, string[] labels)
         public IPropertyValue Convert(IEnumerable<Token> value)
         {
             var list = new List<Token>(value);
-            var options = new IPropertyValue[_labels.Length];
+            var options = new IPropertyValue[_labels.Length == 0 ? 4 : _labels.Length];
 
             if (list.Count == 0) return null;
 
@@ -53,11 +53,11 @@ private sealed class PeriodicValue : IPropertyValue
 
             public PeriodicValue(IPropertyValue[] options, IEnumerable<Token> tokens, string[] labels)
             {
-                Debug.Assert(labels.Length == 2 || labels.Length == 4);
+                Debug.Assert(labels.Length == 0 || labels.Length == 2 || labels.Length == 4);
 
-                _values = new IPropertyValue[labels.Length];
+                _values = new IPropertyValue[labels.Length == 0 ? 4 : labels.Length];
 
-                if (labels.Length == 4)
+                if (_values.Length == 0 || _values.Length == 4)
                 {
                     //Top, right, bottom, left
                     _values[0] = options[0];
@@ -79,7 +79,7 @@ private string[] Values
             {
                 get
                 {
-                    if (_values.Length == 4)
+                    if (_values.Length == 0 || _values.Length == 4)
                     {
                         var top = _values[0].CssText;
                         var right = _values[1].CssText;