Fix NRE in ToCss for a PageRule with no selector

@page rules don't have to have a selector, but calling ToCss on such an instance of PageRule would trigger a NullReferenceException due to SelectorText unconditionally dereferencing Selector.

Fixed by checking if we have a selector or not in ToCss before trying to output it.

Author: lee-m

src/ExCSS.Tests/Cases.cs

@@ -1022,5 +1022,15 @@ public void StylesheetIncludeUnknownDeclarationsWithKnownPropertyShouldNotUseUnk
 
             Assert.IsNotType<UnknownProperty>(((StyleRule)document.Rules[0]).Style.Children.First());
         }
+
+        [Theory]
+        [InlineData("@page { margin-bottom: 5pt; margin-top: 5pt }", "@page {margin-bottom: 5pt; margin-top: 5pt; }")]
+        public void PageRuleCSSOutput(string input, string expected)
+        {
+            var parser = new StylesheetParser();
+            var document = parser.Parse(input);
+
+            Assert.Equal(expected, document.ToCss());
+        }
     }
 }

src/ExCSS/Rules/PageRule.cs

@@ -16,7 +16,7 @@ internal PageRule(StylesheetParser parser)
 
         public override void ToCss(TextWriter writer, IStyleFormatter formatter)
         {
-            writer.Write(formatter.Rule("@page", SelectorText, "{"));
+            writer.Write(formatter.Rule("@page", Selector == null ? "" : SelectorText, "{"));
 
             Style.ToCss(writer, formatter);