How to use NPOI to read Excel spreadsheet that contains empty cells?

Try the GetCell method with the MissingCellPolicy:

ICell cell = row.GetCell(2, MissingCellPolicy.RETURN_NULL_AND_BLANK);

https://github.com/tonyqus/npoi/blob/master/main/HSSF/UserModel/HSSFRow.cs#L444

 /// <summary>
        /// Get the hssfcell representing a given column (logical cell)
        /// 0-based.  If you ask for a cell that is not defined then
        /// you get a null, unless you have set a different
        /// MissingCellPolicy on the base workbook.
        /// </summary>
        /// <param name="cellnum">0 based column number</param>
        /// <returns>Cell representing that column or null if undefined.</returns>
        public ICell GetCell(int cellnum)
        {
            return GetCell(cellnum, book.MissingCellPolicy);
        }

https://github.com/tonyqus/npoi/blob/master/main/HSSF/UserModel/HSSFWorkbook.cs#L429

 [NonSerialized]
        private NPOI.SS.UserModel.MissingCellPolicy missingCellPolicy = NPOI.SS.UserModel.MissingCellPolicy.RETURN_NULL_AND_BLANK;
        /// <summary>
        /// Retrieves the current policy on what to do when
        /// getting missing or blank cells from a row.
        /// The default is to return blank and null cells.
        /// </summary>
        /// <value>The missing cell policy.</value>
        public NPOI.SS.UserModel.MissingCellPolicy MissingCellPolicy
        {
            get { return missingCellPolicy; }
            set { this.missingCellPolicy = value; }
        }

/// <summary>Missing cells are returned as null, Blank cells are returned as normal</summary>
        // Token: 0x04001517 RID: 5399
        public static readonly MissingCellPolicy RETURN_NULL_AND_BLANK = new MissingCellPolicy();

        /// <summary>Missing cells are returned as null, as are blank cells</summary>
        // Token: 0x04001518 RID: 5400
        public static readonly MissingCellPolicy RETURN_BLANK_AS_NULL = new MissingCellPolicy();

        /// <summary>A new, blank cell is Created for missing cells. Blank cells are returned as normal</summary>
        // Token: 0x04001519 RID: 5401
        public static readonly MissingCellPolicy CREATE_NULL_AS_BLANK = new MissingCellPolicy();