Nil worksheet

This fixes a weird problem for me where a spreadsheet opened with the parser has 'nil' entries in the rows array--for example:
```
worksheet.sheet_data.rows.each { |r| p r }
```
returns
```
#<RubyXL::Row:0x007f1906b91c40 @r=2, @spans="7:9", @style_index=nil, @custom_format=false, @ht=18.0, @hidden=nil, @custom_height=true, @outline_level=nil, @collapsed=nil, @thick_top=nil, @thick_bot=true, @ph=nil, @cells=[nil, nil, nil, nil, nil, nil, #<RubyXL::Cell(1,6): "XXX" =#<RubyXL::Formula:0x007f1906b774f8>, datatype = str, style_index = 162>, #<RubyXL::Cell(1,7): nil, datatype = , style_index = 163>, #<RubyXL::Cell(1,8): nil, datatype = , style_index = 164>], @worksheet=#<RubyXL::Sheet:0x007f19066ff470 @name="OMD Userids", @sheet_id=36, @state=nil, @r_id="rId11">>

nil

nil

...

#<RubyXL::Row:0x007f1906b74af0 @r=30, @spans="2:5", @style_index=nil, @custom_format=false, @ht=nil, @hidden=nil, @custom_height=nil, @outline_level=nil, @collapsed=nil, @thick_top=nil, @thick_bot=nil, @ph=nil, @cells=[nil, #<RubyXL::Cell(29,1): "149", datatype = s, style_index = 52>, nil, nil, #<RubyXL::Cell(29,4): nil, datatype = , style_index = 100>], @worksheet=#<RubyXL::Sheet:0x007f19066ff470 @name="OMD Userids", @sheet_id=36, @state=nil, @r_id="rId11">>

```

This makes me unable to add cells or rows anywhere in the spreadsheet (as rows.cells fails with a noMethodError). This fixes that by checking for nils in the array, but the true issue might be in the parser itself.

Thanks for this gem, I'm a huge fan!