Simplify getArgAttrDict/getResultAttrDict by removing unnecessary checks

[mehdi_amini]

Summary

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>() ...

mlir/lib/IR/FunctionSupport.cpp

static bool isEmptyAttrDict(Attribute attr) {
   return attr.cast<DictionaryAttr>().empty();
 }
 
 DictionaryAttr mlir::function_like_impl::getArgAttrDict(Operation *op,
                                                         unsigned index) {
   ArrayAttr attrs = op->getAttrOfType<ArrayAttr>(getArgDictAttrName());
   DictionaryAttr argAttrs =
       attrs ? attrs[index].cast<DictionaryAttr>() : DictionaryAttr();
-  return (argAttrs && !argAttrs.empty()) ? argAttrs : DictionaryAttr();
+  return argAttrs;
 }
 
 DictionaryAttr mlir::function_like_impl::getResultAttrDict(Operation *op,
                                                            unsigned index) {
   ArrayAttr attrs = op->getAttrOfType<ArrayAttr>(getResultDictAttrName());
   DictionaryAttr resAttrs =
       attrs ? attrs[index].cast<DictionaryAttr>() : DictionaryAttr();
-  return (resAttrs && !resAttrs.empty()) ? resAttrs : DictionaryAttr();
+  return resAttrs;
 }
 
 void mlir::function_like_impl::detail::setArgResAttrDict(
     Operation *op, StringRef attrName, unsigned numTotalIndices, unsigned index,
     DictionaryAttr attrs) {
   ArrayAttr allAttrs = op->getAttrOfType<ArrayAttr>(attrName);
   if (!allAttrs) {
     if (attrs.empty())