Fixed templates for column filtering

[thomasp-cnrs]

Original author: @kathibeepboop

fix #347

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.56%. Comparing base (6c3b3dc) to head (53ffabe).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #348   +/-   ##
=======================================
  Coverage   94.56%   94.56%           
=======================================
  Files          37       37           
  Lines         975      975           
=======================================
  Hits          922      922           
  Misses         53       53           

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[MIvanIsten]

Please consider to replace
{% if column.filter.placeholder != null %}
with
{% if column.filter.placeholder is not same as(null) %}
in Filter/select.html.twig

In twig != is not type safe
'' != null => false
'' is not same as(null) => true

[thomasp-cnrs]

@MIvanIsten thanks. I fixed it.