Adjust PDL::Primitive::stats use to avoid issues in 2.068+

[nerdstrike]

A segfault was created in PDL 2.069-2.077 because we are calling the stats() function on empty arrays. A memory allocation change in PDL now causes an error where previously it was working by happy accident. As of PDL 2.078 an exception will be thrown if we attempt this behaviour:

Error in medover:cannot process empty ndarray at ../../blib/lib/PDL/PP.pm line 1065

and this can be seen in t/60-autoqc-results-sequence_error.t where calls to reverse_average_percent_error() invoke the PDL stats function. Compatibility with future versions can be ensured by trapping assignment of undefs into pdl(), as in any case the result would be 0.