[NFC] Set a variable in the mlir data formatter (#65554)

The formatter fails when num_children is invoked and self.impl_type is
not set.

mlir/utils/lldb-scripts/mlirDataFormatters.py

@@ -196,6 +196,7 @@ class AttrTypeSynthProvider:
             valobj, self.abstractVal, internal_dict
         )
         if not self.type:
+            self.impl_type = None
             return
 
         # Grab the ImplTy from the resolved type. This is the 3rd template