A formula cell return an empty formula

[ImportFence]

when a cell copy the formula of above cell, the cell return empty formula. we see the code at line 144 of org.dhatim.fastexcel.reader.RowSpliterator shown as following: the t attribute of f node used "array". But I think it should be both "array" and "shared", because I uncompressed a sample xlsx file Book2.xlsxand checked line 43,"t" is "shared".

Furthermore, a shared f node return formula is empty not null, so I add other logic at line 152.

[ImportFence]

If a cell is empty numeric , the code at 153 is always executed, when we read a sheet with large blank numeric cell, it will be slower and slower.