Fix InputParam required flag silently ignored when a non-None default is set

src/diffusers/modular_pipelines/modular_pipeline.py

@@ -515,16 +515,16 @@ def get_block_state(self, state: PipelineState) -> dict:
         for input_param in state_inputs:
             if input_param.name:
                 value = state.get(input_param.name)
+                if input_param.required and value is None:
+                    raise ValueError(f"Required input '{input_param.name}' is missing")
                 if value is None:
                     # if the value is None (not passed, or passed as None), the first block that reads it sets the
                     # default at call time. For sequential blocks the default is already resolved at compile time
                     # (first declaring block wins, see _get_inputs), but for conditional blocks the selected branch
                     # is only known at runtime: disagreeing branch defaults merge to None (see combine_inputs) and
                     # the block that actually runs applies its own declared default here
                     value = input_param.default
-                if input_param.required and value is None:
-                    raise ValueError(f"Required input '{input_param.name}' is missing")
-                elif value is not None or (value is None and input_param.name not in data):
+                if value is not None or (value is None and input_param.name not in data):
                     data[input_param.name] = value
 
             elif input_param.kwargs_type: