[mlir,python] Fix case when FuncOp.arg_attrs is not set (#117188)

FuncOps can have `arg_attrs`, an array of dictionary attributes
associated with their arguments.

E.g., 

```mlir
func.func @main(%arg0: tensor<8xf32> {test.attr_name = "value"}, %arg1: tensor<8x16xf32>)
```

These are exposed via the MLIR Python bindings with
`my_funcop.arg_attrs`.

In this case, it would return `[{test.attr_name = "value"}, {}]`, i.e.,
`%arg1` has an empty `DictAttr`.

However, if I try and access this property from a FuncOp with an empty
`arg_attrs`, e.g.,

```mlir
func.func @main(%arg0: tensor<8xf32>, %arg1: tensor<8x16xf32>)
```

This raises the error:

```python
    return ArrayAttr(self.attributes[ARGUMENT_ATTRIBUTE_NAME])
                     ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
KeyError: 'attempt to access a non-existent attribute'
```

This PR fixes this by returning the expected `[{}, {}]`.

Author: Wheest

mlir/python/mlir/dialects/func.py

@@ -105,6 +105,8 @@ def add_entry_block(self, arg_locs: Optional[Sequence[Location]] = None):
 
     @property
     def arg_attrs(self):
+        if ARGUMENT_ATTRIBUTE_NAME not in self.attributes:
+            return ArrayAttr.get([DictAttr.get({}) for _ in self.type.inputs])
         return ArrayAttr(self.attributes[ARGUMENT_ATTRIBUTE_NAME])
 
     @arg_attrs.setter