Simplify getArgAttrDict/getResultAttrDict by removing unnecessary checks

There is a slight change in behavior: if the arg dictionnary is empty
then we return this empty dictionnary instead of a null attribute.
This is more consistent with accessing it through:

  ArrayAttr args_attr = func_op.getAllArgAttrs();
  args_attr[num].cast<DictionnaryAttr>() ...

Differential Revision: https://reviews.llvm.org/D104189