Table parse crashes when file (data frame) is empty

[vreuter]

The aggression of the ImageHandler initialization, in seeking and attempting to parse any and all contents of the analysis subfolder, means that if one of the steps results in an empty table file, e.g. when the ROI merge threshold is set to 0 for no-merge behavior, and then therefore the merge contributors CSV file is empty, the parse attempt will fail with a pandas.errors.EmptyDataError. This should be protected against.

First seen with @josephneos1010 in 6305

Related:

• Forgetting pixelSeparationBeneathWhichSpotRoisWillMerge key in the parameters configuration file fails to be caught in pipeline precheck #413 (because we're allowing the parameter value related to that ticket to be set to 0, which will cause a specific instance of the behavior of this ticket)
• ImageHandler aggression: parsing contents of analysis subfolder #411 (because it's the parsing aggression described in that ticket which leads to the behavior described in this ticket)