Let null MockFileData create empty file (#360)

Fixes #356

Author: LosManos

System.IO.Abstractions.TestingHelpers.Tests/MockFileSystemTests.cs

@@ -54,6 +54,19 @@ public void MockFileSystem_GetFile_ShouldReturnFileRegisteredInConstructorWhenPa
             Assert.AreEqual(file1, result);
         }
 
+        [Test]
+        public void MockFileSystem_AddFile_ShouldHandleNullFileDataAsEmpty()
+        {
+            var fileSystem = new MockFileSystem(new Dictionary<string, MockFileData>
+            {
+                { @"c:\something\nullish.txt", null }
+            });
+
+            var result = fileSystem.File.ReadAllText(@"c:\SomeThing\nullish.txt");
+
+            Assert.IsEmpty(result, "Null MockFileData should be allowed for and result in an empty file.");
+        }
+
         [Test]
         public void MockFileSystem_AddFile_ShouldRepaceExistingFile()
         {

System.IO.Abstractions.TestingHelpers/MockFileSystem.cs

@@ -138,7 +138,7 @@ public void AddFile(string path, MockFileData mockFile)
                     AddDirectory(directoryPath);
                 }
 
-                SetEntry(fixedPath, mockFile);
+                SetEntry(fixedPath, mockFile ?? new MockFileData(string.Empty));
             }
         }