Handle null returns in CivitAI base model type fallback chain

- Chain '?? []' so an all-null fallback can't NRE on Count
- Null-conditional access on enumsResponse in case the API returns 204

Addresses gemini-code-assist review comments on LykosAI#1239.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mohnjiles

StabilityMatrix.Avalonia/Services/CivitBaseModelTypeService.cs

@@ -99,7 +99,7 @@ public async Task<List<string>> GetBaseModelTypes(bool forceRefresh = false, boo
             if (civitBaseModels.Count <= 0)
             {
                 civitBaseModels =
-                    await TryGetBaseModelsFromEnumsEndpoint() ?? await TryGetLegacyBaseModelList();
+                    await TryGetBaseModelsFromEnumsEndpoint() ?? await TryGetLegacyBaseModelList() ?? [];
 
                 civitBaseModels =
                     civitBaseModels.Count > 0 ? civitBaseModels : GetKnownVisibleBaseModelTypes();
@@ -143,7 +143,7 @@ public void ClearCache()
         try
         {
             var enumsResponse = await civitApi.GetEnums();
-            var baseModels = enumsResponse.ActiveBaseModel ?? enumsResponse.BaseModel;
+            var baseModels = enumsResponse?.ActiveBaseModel ?? enumsResponse?.BaseModel;
 
             if (baseModels is { Count: > 0 })
             {