avoid NaN in (un)selected.marker.opacity

NaN !== NaN wat was breaking Plotly.react.

Author: alexcjohnson

src/lib/coerce.js

@@ -406,6 +406,9 @@ exports.coerceSelectionMarkerOpacity = function(traceOut, coerce) {
     if(!traceOut.marker) return;
 
     var mo = traceOut.marker.opacity;
+    // you can still have a `marker` container with no markers if there's text
+    if(mo === undefined) return;
+
     var smoDflt;
     var usmoDflt;