Fix crash when attempting to update a model (#4192)

## What type of PR is this? (check all applicable)

- [X] Bug Fix


## Have you discussed this change with the InvokeAI team?
- [X No, because small fix

      
## Have you updated all relevant documentation?
- [X] Yes

## Description

A logic bug was introduced in PR #4109 that caused Web-based model
updates to fail with a pydantic validation error. This corrects the
problem.

## Related Tickets & Documents

PR #4109

Author: lstein

invokeai/backend/model_management/model_manager.py

@@ -595,9 +595,10 @@ def list_model(
         the combined format of the list_models() method.
         """
         models = self.list_models(base_model, model_type, model_name)
-        if len(models) > 1:
+        if len(models) >= 1:
             return models[0]
-        return None
+        else:
+            return None
 
     def list_models(
         self,