Opening an empty or whitespace-only .coder.yaml throws ArgumentException out of DocumentStore.Load and can take the editor down #121

What's wrong

DocumentStore.Load says it returns every failure rather than throwing, because an exception that escapes a draw call takes the application down. An empty file breaks that promise:

• Coder.Editor/DocumentStore.cs:68 calls deserializer.Deserialize(yaml).
• That call is guarded only by catch (YamlException) (line 70) and catch (InvalidOperationException) (line 76).
• Coder/Serialization/YamlDeserializer.cs:39 starts with ArgumentException.ThrowIfNullOrWhiteSpace(yaml);.
• ArgumentException is neither of the caught types, so it propagates out of Load.
• The caller, CoderEditorApp.Open (Coder.Editor/CoderEditorApp.cs:589), is called from the ImGui menu, both File > Open (line 348) and the recent-files list (line 367). Neither caller has a try/catch.

Failure scenario

Open a 0-byte file, or a file containing only " \n", from File > Open or from the recent-files list. A 0-byte file is an ordinary thing to meet: an interrupted save or a newly created file leaves one. Load throws System.ArgumentException instead of returning DocumentResult.Failed(...), and the exception escapes the frame.

This was confirmed against the built ktsu.Coder.dll: Deserialize("") and Deserialize(" \n") both throw System.ArgumentException. The existing tests, CoderEditorAppTests.Open_ReportsAMissingFile and Open_ReportsAFileThatIsNotADocument, don't cover empty content.

Suggested fix

• In DocumentStore.Load, check string.IsNullOrWhiteSpace(yaml) before deserializing, and return DocumentResult.Failed($"{path} is empty."). Alternatively, add an ArgumentException catch alongside the other two.
• Add Open_ReportsAnEmptyFile, covering both empty and whitespace-only content, and assert that Open returns false and sets Status.