fix tests

Author: Ombrelin

src/PlexRichPresence.Core/PlexSession.cs

@@ -12,7 +12,7 @@ public record PlexSession(
     PlexMediaType MediaType,
     long Duration,
     long ViewOffset,
-    string Thumbnail
+    string? Thumbnail
 )
 {
     public PlexSession() : this(

src/PlexRichPresence.PlexActivity/PlexSessionMapper.cs

@@ -18,7 +18,7 @@ public PlexSession Map(SessionMetadata metadata, string plexServerHost, string p
             GetMediaType(metadata.Type),
             metadata.Duration,
             metadata.ViewOffset,
-            $"{plexServerHost}{metadata.Thumb[1..]}?X-Plex-Token={plexToken}"
+            metadata.Thumb is null ? null : $"{plexServerHost}{metadata.Thumb[1..]}?X-Plex-Token={plexToken}"
         );
 
     public PlexSession Map(Metadata metadata, string state, long viewOffset, string plexServerHost, string plexToken) =>
@@ -32,7 +32,7 @@ public PlexSession Map(Metadata metadata, string state, long viewOffset, string
             GetMediaType(metadata.Type),
             metadata.Duration,
             viewOffset,
-            $"{plexServerHost}{metadata.Thumb[1..]}?X-Plex-Token={plexToken}"
+            metadata.Thumb is null ? null : $"{plexServerHost}{metadata.Thumb[1..]}?X-Plex-Token={plexToken}"
         );