Add null check for plugin metadata deserialization

Throw a JsonException if deserializing plugin metadata from JSON returns null. This prevents null metadata from being used and improves error handling for invalid or corrupted plugin.json files.

Author: Jack251970

Flow.Launcher.Core/Plugin/PluginManager.cs

@@ -899,7 +899,8 @@ internal static bool InstallPlugin(UserPlugin plugin, string zipFilePath, bool c
             PluginMetadata newMetadata;
             try
             {
-                newMetadata = JsonSerializer.Deserialize<PluginMetadata>(File.ReadAllText(metadataJsonFilePath));
+                newMetadata = JsonSerializer.Deserialize<PluginMetadata>(File.ReadAllText(metadataJsonFilePath)) ??
+                    throw new JsonException("Deserialized metadata is null");
             }
             catch (Exception ex)
             {