Arm backend: Handle None arg for creating empty TosaArg (#12952)

### Summary
In order to get the same number and order of arguments from fx.Node.args
and the list[TosaArg] used in nodevisitor, we need to allow for empty
(None) args. The empty TosaArg is represented with fields set to None
and empty string for name.

Author: per

backends/arm/tosa_mapping.py

@@ -102,8 +102,6 @@ def __process_number(self, argument: float | int):
     def __init__(
         self, argument: Any, tosa_spec: Optional[TosaSpecification] = None
     ) -> None:
-        if argument is None:
-            return
         if tosa_spec is None:
             raise ValueError("tosa_spec is None")
         elif not isinstance(tosa_spec, TosaSpecification):
@@ -125,6 +123,13 @@ def __init__(
             # Dtype is parsed from fake tensor
             return
 
+        if argument is None:
+            self.name = ""
+            self.dtype = None
+            self.shape = None
+            self.dim_order = None
+            return
+
         raise RuntimeError(
             f"Unhandled node input argument: {argument}, of type {type(argument)}"
         )