feat: add Null to MockFileStream to shadow the static Stream.Null field (#1025)

Fixes #1009 

This is a suggestion based on the [workaround by @sheila-stewart](#1009 (comment)).

Author: vbreuss

src/TestableIO.System.IO.Abstractions.TestingHelpers/MockFileStream.cs

@@ -9,6 +9,24 @@ namespace System.IO.Abstractions.TestingHelpers
     [Serializable]
     public class MockFileStream : FileSystemStream, IFileSystemAclSupport
     {
+        /// <summary>
+        ///     Wrapper around a <see cref="Stream" /> with no backing store, which
+        ///     is used as a replacement for a <see cref="FileSystemStream" />. As such
+        ///     it implements the same properties and methods as a <see cref="FileSystemStream" />.
+        /// </summary>
+        public new static FileSystemStream Null { get; } = new NullFileSystemStream();
+
+        private class NullFileSystemStream : FileSystemStream
+        {
+            /// <summary>
+            /// Initializes a new instance of <see cref="NullFileSystemStream" />.
+            /// </summary>
+            public NullFileSystemStream() : base(Null, ".", true)
+            {
+                
+            }
+        }
+
         private readonly IMockFileDataAccessor mockFileDataAccessor;
         private readonly string path;
         private readonly FileAccess access = FileAccess.ReadWrite;

tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockFileStreamTests.cs

@@ -226,5 +226,24 @@ public void MockFileStream_FlushBool_ShouldNotChangePosition([Values] bool flush
                 Assert.AreEqual(200, stream.Position);
             }
         }
+
+        [Test]
+        public void MockFileStream_Null_ShouldReturnSingletonObject()
+        {
+            var result1 = MockFileStream.Null;
+            var result2 = MockFileStream.Null;
+
+            Assert.AreSame(result1, result2);
+        }
+
+        [Test]
+        public void MockFileStream_Null_ShouldHaveExpectedProperties()
+        {
+            var result = MockFileStream.Null;
+
+            Assert.AreEqual(result.Name, ".");
+            Assert.AreEqual(result.Length, 0);
+            Assert.AreEqual(result.IsAsync, true);
+        }
     }
 }