[MLIR] Add ArrayAttr::empty()

[jurahul]

Summary

Add ArrayAttr::empty() to check for an empty ArrayAttr

mlir/include/mlir/IR/Attributes.h

public:
   ArrayRef<Attribute> getValue() const;
   Attribute operator[](unsigned idx) const;
 
   /// Support range iteration.
   using iterator = llvm::ArrayRef<Attribute>::iterator;
   iterator begin() const { return getValue().begin(); }
   iterator end() const { return getValue().end(); }
   size_t size() const { return getValue().size(); }
+  bool empty() const { return size() == 0; }
 
   /// Methods for support type inquiry through isa, cast, and dyn_cast.
   static bool kindof(unsigned kind) {
     return kind == StandardAttributes::Array;
   }
 
 private:
   /// Class for underlying value iterator support.