Fix/import excel error with nil header cells

[sinaeftekhar]

Sometimes the excel files might contain nil values which is not evident to the users eye, but messes with the app.
This fix solves this problem.
Fixes issue #91

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (abbf0c6) 96.79% compared to head (90e08d6) 96.71%.
Report is 7 commits behind head on master.

Files	Patch %	Lines
lib/decidim/term_customizer/import/readers/xlsx.rb	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
- Coverage   96.79%   96.71%   -0.08%     
==========================================
  Files          58       58              
  Lines        1153     1157       +4     
==========================================
+ Hits         1116     1119       +3     
- Misses         37       38       +1     

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