worksheet: Add method to check if the worksheet is empty

A worksheet is considered empty if it doesn't have any cells.

include/xlnt/worksheet/worksheet.hpp

@@ -770,6 +770,12 @@ public:
     /// </summary>
     bool has_drawing() const;
 
+    /// <summary>
+    /// Returns true if this worksheet is empty.
+    /// A worksheet is considered empty if it doesn't have any cells.
+    /// </summary>
+    bool is_empty() const;
+
 private:
     friend class cell;
     friend class const_range_iterator;

source/worksheet/worksheet.cpp

@@ -1335,4 +1335,9 @@ bool worksheet::has_drawing() const
     return d_->drawing_.is_set();
 }
 
+bool worksheet::is_empty() const
+{
+    return d_->cell_map_.empty();
+}
+
 } // namespace xlnt