Report an empty document as empty rather than unparseable [patch]

Coder.Editor/DocumentStore.cs

@@ -61,6 +61,13 @@ public DocumentResult Load(string path)
 			return DocumentResult.Failed($"Could not read {path}: {ex.Message}");
 		}
 
+		// An interrupted save or a newly created file leaves nothing in it, and the deserializer
+		// refuses empty text as a bad argument, which would read to the user as a parse failure.
+		if (string.IsNullOrWhiteSpace(yaml))
+		{
+			return DocumentResult.Failed($"{path} is empty.");
+		}
+
 		AstNode? root;
 		try
 		{

Coder.Test/Editor/CoderEditorAppTests.cs

@@ -235,6 +235,27 @@ public void Open_ReportsAFileThatIsNotADocument()
 		Assert.IsNull(app.DocumentPath);
 	}
 
+	/// <summary>
+	/// Tests that an empty or whitespace-only file is reported as empty rather than as a document
+	/// the editor cannot parse, since an interrupted save leaves exactly that behind.
+	/// </summary>
+	/// <param name="content">What the file holds.</param>
+	[TestMethod]
+	[DataRow("")]
+	[DataRow("  \n")]
+	public void Open_ReportsAnEmptyFile(string content)
+	{
+		DocumentStore store = NewStore();
+		string empty = Path.Combine(root, "empty" + DocumentStore.Extension);
+		File.WriteAllText(empty, content);
+		CoderEditorApp app = NewApp(store);
+
+		Assert.IsFalse(app.Open(empty));
+
+		Assert.AreEqual($"{empty} is empty.", app.Status);
+		Assert.IsNull(app.DocumentPath);
+	}
+
 	/// <summary>
 	/// Tests that a write the filesystem refuses is reported rather than thrown, so a bad
 	/// destination cannot take the editor down mid-session.