Harden reselect against empty/undefined calcdata entries

[TheSisb]

Skip empty/undefined calcdata entries in epmtySplomSelectionBatch and determineSearchTraces instead of dereferencing them.

What happens

reselect runs unconditionally at the end of every redraw sequence (newPlot / react / relayout / resize). Its helper epmtySplomSelectionBatch iterates gd.calcdata and dereferences cd[i][0] with no guard:

  for (var i = 0; i < cd.length; i++) {
      var cd0 = cd[i][0];   // throws if cd[i] is undefined
      var trace = cd0.trace;

When gd.calcdata contains an empty ([]) or undefined entry, this throws
and the redraw promise rejects:

  TypeError: Cannot read properties of undefined (reading '0')
      at epmtySplomSelectionBatch (src/components/selections/select.js)
      at reselect             (src/components/selections/select.js)
      at Plots.reselect       (src/plots/plots.js)
      at Lib.syncOrAsync      (src/lib/index.js)
      at relayout             (src/plot_api/plot_api.js)