Ensure blank cells are returned as 'nil' instead of being omitted

This is to fix an issue when reading a spreadsheet with blank values. The iterator behaviour from POI skips blank rows and cells.

Intend to read tabular data and just zipmap the header row with remaining but any rows containing blank cells ended up with mismatched data.

Example
---
<table>
<tr>
<td>Col 1</td>
<td>Col 2</td>
</tr>
<tr>
<td>Row 1 Val 1</td>
<td>Row 1 Val 2</td>
</tr>
<tr>
<td></td>
<td>Row 2 Val 2</td>
</tr>
</table>

Would end up as 
```
[
{"Col 1" "Row 1 Val 1" "Col 2" "Row 1 Val 2" }
{"Col 1" "Row 1 Val 2" "Col 2" nil}
]
```

instead of 
```
[
{"Col 1" "Row 1 Val 1" "Col 2" "Row 1 Val 2" }
{"Col 1" nil "Col 2" "Row 1 Val 2"}
]
```