Allow pre-defined empty column types

[ArslanaWu]

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Description

Fix issue #822 by letting user specify the type of empty column in advance.

Testing

Add columns-with-empty-column.xlsx, which has one empty column.

Add two test in XlsxReaderTest.java

• testColumnsWithEmptyColumn()
  Determine whether the empty columns have not been created when the user does not specify the type of empty columns
• testColumnsWithEmptyColumnAndPreserve()
  Determine whether the empty columns are created and the type is correct when the user specifies the type of empty columns

[lwhite1]

I'm not sure I agree with the design here for two reasons:

• I suspect that the existing CsvReadOption columnTypes method could handle this use case if it were added to the Excel read options (or better, to the ReadOption base class).
• If that's not the case, then this method should be generalized so that it covers the other file types.

Between the two , I have a strong preference for using the columnTypes method unless that's not feasible for some reason.

[ArslanaWu]

I think you are right, sorry for disturbing.