filter out empty plotly_selected events

[alexcjohnson]

plotly.js emits plotly_selected events with empty data on single click - odd behavior but it has been kept for backward compatibility. In Dash these are filtered out (that's the if (!isNil(selected)). I'm just replicating that behavior here, so that an empty selected_data param truly means there's no selection.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.20%. Comparing base (32b147d) to head (8d08ac5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7312   +/-   ##
=======================================
  Coverage   82.19%   82.20%           
=======================================
  Files         337      337           
  Lines       50394    50394           
=======================================
+ Hits        41422    41424    +2     
+ Misses       8972     8970    -2     

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[philippjfr]

Good catch, thank you!