Fix panic in removeEmptySeries if series is nil and xFilesFactor specified

[carrieedwards]

Describe the bug
A panic has been observed when the removeEmptySeries function is passed a series that is nil/a length of 0, and an xFilesFactor has also been specified.

The panic occurs here:

args[0].XFilesFactor

Because args[0] is nil, trying to access the XFilesFactor setting results in runtime error: index out of range [0] with length 0.

To replicate, pass in a series that does not exist as an argument, as well as any xFilesFactor values:

removeEmptySeries(nonexistantSeries, 0.5)