Logic fixes and allow empty attribute values to render as such

Author: donker

DNN Platform/DotNetNuke.Web.Client.ResourceManager/Models/FontResource.cs

@@ -37,7 +37,7 @@ public override string Render(int crmVersion, bool useCdn, string applicationPat
         {
             htmlString.Append($" rel=\"preload\" as=\"font\"");
         }
-        else if (!this.Attributes.ContainsKey("rel"))
+        else if (!this.Preload && !this.Attributes.ContainsKey("rel"))
         {
             htmlString.Append($" rel=\"font\"");
         }

DNN Platform/DotNetNuke.Web.Client.ResourceManager/Models/ResourceBase.cs

@@ -239,7 +239,7 @@ protected void RenderAttributes(StringBuilder htmlString)
         {
             foreach (var attribute in this.Attributes)
             {
-                if (string.IsNullOrEmpty(attribute.Value))
+                if (attribute.Value is null)
                 {
                     htmlString.Append($" {attribute.Key}");
                 }

DNN Platform/DotNetNuke.Web.Client.ResourceManager/Models/StylesheetResource.cs

@@ -41,7 +41,7 @@ public override string Render(int crmVersion, bool useCdn, string applicationPat
         {
             htmlString.Append($" rel=\"preload\" as=\"style\"");
         }
-        else if (!this.Attributes.ContainsKey("rel"))
+        else if (!this.Preload && !this.Attributes.ContainsKey("rel"))
         {
             htmlString.Append($" rel=\"stylesheet\"");
         }

DNN Platform/DotNetNuke.Web.Client/Controls/ClientResourceInclude.cs

@@ -139,7 +139,7 @@ private static void ParseHtmlAttributesIntoDictionary(string attributes, IDictio
                 if (isKey && c == ' ')
                 {
                     // this means we have a key without a value, we should add it to the dictionary with an empty value and start a new key
-                    destination[key] = string.Empty;
+                    destination[key] = null;
                     key = string.Empty;
                     continue;
                 }