[Chart] Empty values in chart creates an index overflow in data values array

[guillaume-ro-fr]

This is:

- [x] a bug report

What is the expected behavior?

When a chart have the first of its values at null, the evaluation of the multi-level series must pass.

What is the current behavior?

Currently, detection is based on the 0 index, not the first index of the array.

What are the steps to reproduce?

I don't know if we can reproduce it without an existing spreadsheet from Excel : even with null values, PhpSpreadsheet fills the values, without leaving a gap in the indices in the XML (?).

Which versions of PhpSpreadsheet and PHP are affected?

PhpSpreadsheet 1.5.0
PHP 7.1