Add tests for previously-uncovered SparklineGroup setters (axis types,
manual limits, additional display options and colours) and a hand-crafted
Xlsx fixture exercising the reader's edge cases (non-sparkline ext, group
without sparklines, sparkline with empty sqref, missing colour elements).
Harden Reader/Xlsx/Sparklines against malformed input: guard the
sparklineGroup and sparkline iterations against a null children set so an
empty <x14:sparklineGroups> or <x14:sparklines> element no longer emits a
PHP warning.
saifulislamferoz