fixed Llama._create_completion suffix check, it can be either None or str instance (#854)

llama_cpp/llama.py

@@ -921,6 +921,7 @@ class Llama:
         grammar: Optional[LlamaGrammar] = None,
     ) -> Union[Iterator[Completion], Iterator[CompletionChunk]]:
         assert self.ctx is not None
+        assert suffix is None or suffix.__class__ is str
 
         completion_id: str = f"cmpl-{str(uuid.uuid4())}"
         created: int = int(time.time())