Reading a preset without an Info chunk returns MetaInfo "" instead of null, so each read/write round trip adds an empty Info chunk #14

What's wrong

VstPresetFile finishes reading with new VstPreset(classId, component, controller, metaInfo ?? string.Empty, version) (EffectsHost.Plugin/VstPresetFile.cs:269).

That contradicts the documented contract of VstPreset.MetaInfo: null when the container has no such chunk, where an empty value is a different thing. It also contradicts the OptionalChunksRoundTrip test's summary ("reports their absence"). That test only asserts ControllerState is null, so the regression is not caught.

Repro

At 936acc2:

1. Write a preset with MetaInfo: null. The file is 77 bytes.
2. Read it back: MetaInfo == "".
3. Write it again. The file is now 97 bytes, because a zero-length Info chunk has been added.

A preset loaded and re-saved by the plugin therefore no longer matches the original bytes. A host or tool that treats "has an Info chunk" as meaningful sees a difference that the user never made.

Suggested fix

• Pass metaInfo through without ?? string.Empty.
• Add Assert.IsNull(restored.MetaInfo) to OptionalChunksRoundTrip.

Acceptance criteria

• A preset written without meta info reads back with MetaInfo == null.
• Re-writing it produces byte-identical output.