Fix IDE0041: Use 'is null' check

Author: bkoelman

src/JsonApiDotNetCore/Configuration/PageNumber.cs

@@ -25,7 +25,7 @@ public PageNumber(int oneBasedValue)
 
     public bool Equals(PageNumber? other)
     {
-        if (ReferenceEquals(null, other))
+        if (other is null)
         {
             return false;
         }

src/JsonApiDotNetCore/Configuration/PageSize.cs

@@ -23,7 +23,7 @@ public PageSize(int value)
 
     public bool Equals(PageSize? other)
     {
-        if (ReferenceEquals(null, other))
+        if (other is null)
         {
             return false;
         }

src/JsonApiDotNetCore/Serialization/Response/ResourceObjectTreeNode.cs

@@ -210,7 +210,7 @@ private void AssertIsTreeRoot()
 
     public bool Equals(ResourceObjectTreeNode? other)
     {
-        if (ReferenceEquals(null, other))
+        if (other is null)
         {
             return false;
         }