Minor optimisations

Fixes serilog#241

Author: jods4

src/Serilog.Extensions.Logging/Extensions/Logging/SerilogLogger.cs

@@ -103,12 +103,12 @@ LogEvent PrepareWrite<TState>(LogEventLevel level, EventId eventId, TState state
                 {
                     messageTemplate = value;
                 }
-                else if (property.Key.StartsWith("@"))
+                else if (property.Key.StartsWith('@'))
                 {
                     if (_logger.BindProperty(GetKeyWithoutFirstSymbol(DestructureDictionary, property.Key), property.Value, true, out var destructured))
                         properties.Add(destructured);
                 }
-                else if (property.Key.StartsWith("$"))
+                else if (property.Key.StartsWith('$'))
                 {
                     if (_logger.BindProperty(GetKeyWithoutFirstSymbol(StringifyDictionary, property.Key), property.Value?.ToString(), true, out var stringified))
                         properties.Add(stringified);
@@ -167,10 +167,10 @@ LogEvent PrepareWrite<TState>(LogEventLevel level, EventId eventId, TState state
 
     static object? AsLoggableValue<TState>(TState state, Func<TState, Exception?, string>? formatter)
     {
-        object? stateObj = state;
+        object? stateObj = null;
         if (formatter != null)
             stateObj = formatter(state, null);
-        return stateObj;
+        return stateObj ?? state;
     }
 
     internal static LogEventProperty CreateEventIdProperty(EventId eventId)