phpgraphlib_pie > Fix for last value (0)

When the last value of the data array is 0, entire pie chart is filled with the last data because the an arc from (360 -> 360) is created.
This problem can be fixed by simply skipping the drawing of the arcs when a value of (0) is provided.
Merge pull request #6 from LupinWonse/patch-1

phpgraphlib_pie > Fix for last value (0)