Consider free symbols in SDFG init and exit code, fix None vs. empty …

…logic bug

dace/sdfg/sdfg.py

@@ -1341,6 +1341,12 @@ def used_symbols(self, all_symbols: bool) -> Set[str]:
 
         defined_syms |= set(self.constants_prop.keys())
 
+        # Add used symbols from init and exit code
+        for code in self.init_code.values():
+            free_syms |= symbolic.symbols_in_code(code.as_string, self.symbols.keys())
+        for code in self.exit_code.values():
+            free_syms |= symbolic.symbols_in_code(code.as_string, self.symbols.keys())
+
         # Add free state symbols
         used_before_assignment = set()
 
@@ -1472,7 +1478,7 @@ def init_signature(self, for_call=False, free_symbols=None) -> str:
             :param for_call: If True, returns arguments that can be used when calling the SDFG.
         """
         # Get global free symbols scalar arguments
-        free_symbols = free_symbols or self.free_symbols
+        free_symbols = free_symbols if free_symbols is not None else self.used_symbols(all_symbols=False)
         return ", ".join(
             dt.Scalar(self.symbols[k]).as_arg(name=k, with_types=not for_call, for_call=for_call)
             for k in sorted(free_symbols) if not k.startswith('__dace'))