Fix for #102 GetFiles needs to return empty instead of null

Author: Jeavon

src/UmbracoFileSystemProviders.Azure.Tests/AzureBlobFileSystemTestsBase.cs

@@ -365,7 +365,7 @@ public void TestGetFilesInvalidPath()
             IEnumerable<string> actual = provider.GetFiles("/somethingmissing", "*.jpg");
 
             // Assert
-            Assert.IsNull(actual);
+            Assert.IsEmpty(actual);
         }
 
         /// <summary>

src/UmbracoFileSystemProviders.Azure/AzureFileSystem.cs

@@ -466,7 +466,7 @@ public IEnumerable<string> GetFiles(string path, string filter)
             if (!blobList.Any())
             {
                 this.LogHelper.Error<AzureFileSystem>("Blob not found", new DirectoryNotFoundException($"Blob not found at '{path}'"));
-                return null;
+                return Enumerable.Empty<string>();
             }
 
             return blobList.OfType<CloudBlockBlob>().Select(cd =>