AssetIconAttribute on method not working because of a validation bug (fix included)

When applied to a parameterless method, logging "method should have no parameters" error. Figured it's the bug in the condition of validation. In `GraphicSourceSelector.cs`, line 108:

```csharp
		private static bool IsSupportedParameters(Type[] parameterTypes)
		{
			return parameterTypes.Length != 0;
		}
```

should be

```csharp
			return parameterTypes.Length == 0;
```