Handle null values in cells.

[martijn]

It turns out Excel completely skips the <c /> element for cells without a value as illustrated in cell D3 and E3 in the example below. This calls for some more sophisticated row loading

<row r="3" spans="1:18" x14ac:dyDescent="0.3">
  <c r="A3" s="4" t="str"><f t="shared" si="0"/><v>ABCD</v></c>
  <c r="B3" t="s"><v>97</v></c>
  <c r="C3"><v>16</v></c>
  <c r="F3" t="s"><v>147</v></c>
  <c r="G3" t="s"><v>8</v></c>