Merge pull request #160 from meziantou/fix-nre

Fix NullReferenceException in ChangeDeclarations

src/AngleSharp.Css/Dom/Internal/CssStyleDeclaration.cs

@@ -377,6 +377,11 @@ namespace AngleSharp.Css.Dom
 
         private void ChangeDeclarations(IEnumerable<ICssProperty> decls, Predicate<ICssProperty> defaultSkip, Func<ICssProperty, ICssProperty, Boolean> removeExisting)
         {
+            if (decls is null)
+            {
+                return;
+            }
+
             var declarations = new List<ICssProperty>();
 
             foreach (var newdecl in decls)