poor demux summarize error message when summarizing empty demux

[ChrisKeefe]

Bug Description When qiime demux summarize is run on an empty 'demux' Artifact, it produces a sub-optimal error message:

Plugin error from demux: Cannot describe a DataFrame without columns

This error message is likely being passed up from Pandas.

Steps to reproduce the behavior

1. Download or create an empty SampleData[SequencesWithQuality] or other acceptable Artifact. (e.g. this one)
2. Pass it as an input to qiime demux summarize

Expected behavior demux summarize should test for an empty Artifact and render a clearer error message. e.g. 'Input data contains no sequences.'

Screenshots

Computation Environment

• OS: linuxmint 18.3
• QIIME 2 Release 2019.4

References demux summarize source