[MLIR][NFC] Add SymbolUse::UseRange::empty()

mlir/include/mlir/IR/SymbolTable.h

public:
   /// This class implements a range of SymbolRef uses.
   class UseRange {
   public:
     UseRange(std::vector<SymbolUse> &&uses) : uses(std::move(uses)) {}
 
     using iterator = std::vector<SymbolUse>::const_iterator;
     iterator begin() const { return uses.begin(); }
     iterator end() const { return uses.end(); }
+    bool empty() const { return uses.empty(); }
 
   private:
     std::vector<SymbolUse> uses;
   };
 
   /// Get an iterator range for all of the uses, for any symbol, that are nested
   /// within the given operation 'from'. This does not traverse into any nested
   /// symbol tables. This function returns None if there are any unknown