PartitionKey: Fixes NullRef in toString handling for None for PartitionKey.ToString()

Microsoft.Azure.Cosmos/src/FeedRange/FeedRanges/FeedRangePartitionKey.cs

@@ -79,7 +79,7 @@ public override string ToString()
         {
             if (this.PartitionKey.IsNone)
             {
-                return "None";
+                return PartitionKey.NoneString;
             }
 
             return this.PartitionKey.InternalKey.ToJsonString();

Microsoft.Azure.Cosmos/src/PartitionKey.cs

@@ -48,6 +48,11 @@ namespace Microsoft.Azure.Cosmos
         /// </summary>
         internal bool IsNone { get; }
 
+        /// <summary>
+        /// String representation of None for use when representing Partition Key as none.
+        /// </summary>
+        internal const string NoneString = "None";
+
         /// <summary>
         /// Creates a new partition key value.
         /// </summary>
@@ -173,6 +178,11 @@ public bool Equals(PartitionKey other)
         /// <returns>The string representation of the partition key value</returns>
         public override string ToString()
         {
+            if (this.IsNone)
+            {
+                return NoneString;
+            }
+
             if (this.InternalKey == null)
             {
                 return PartitionKey.NullPartitionKeyInternal.ToJsonString();

Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/PartitionKeyTests.cs

@@ -88,6 +88,15 @@ public void TestPartitionKeyDefinitionAreEquivalent()
             Assert.IsTrue(PartitionKeyDefinition.AreEquivalent(definition1, definition2));
         }
 
+        [TestMethod]
+        public void NoneToStringNotNullRef()
+        {
+            const string noneString = "None";
+            Cosmos.PartitionKey noneKey = Cosmos.PartitionKey.None;
+
+            Assert.AreEqual(noneString, noneKey.ToString());
+        }
+
         [TestMethod]
         public void RoundTripTests()
         {