[Fix] Treat an empty grammar constraint as unset in SamplingParams (#33328)

python/sglang/srt/constrained/grammar_manager.py

@@ -147,7 +147,7 @@ class GrammarManager:
                     key = ("regex", req.sampling_params.regex)
                 elif req.sampling_params.ebnf is not None:
                     key = ("ebnf", req.sampling_params.ebnf)
-                elif req.sampling_params.structural_tag:
+                elif req.sampling_params.structural_tag is not None:
                     key = ("structural_tag", req.sampling_params.structural_tag)
 
                 value, cache_hit = self.grammar_backend.get_cached_or_future_value(

python/sglang/srt/sampling/sampling_params.py

@@ -134,6 +134,12 @@ class SamplingParams(msgspec.Struct, kw_only=True, array_like=True):
             self.no_stop_trim if self.no_stop_trim is not None else False
         )
 
+        # An empty grammar constraint means "unset", not "constrain to nothing".
+        self.json_schema = self.json_schema or None
+        self.regex = self.regex or None
+        self.ebnf = self.ebnf or None
+        self.structural_tag = self.structural_tag or None
+
         # Process some special cases
         if 0 <= self.temperature < _SAMPLING_EPS:
             # top_k = 1 means greedy sampling