Allow empty attributes

[bersace]

Closes: #282

cc @meegoSVK, @andersbogsnes.

[codecov-commenter]

Codecov Report

Merging #336 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #336      +/-   ##
==========================================
+ Coverage   99.63%   99.68%   +0.05%     
==========================================
  Files          12       12              
  Lines        1906     1914       +8     
==========================================
+ Hits         1899     1908       +9     
+ Misses          7        6       -1     

Impacted Files	Coverage Δ
ldap2pg/manager.py	100.00% <100.00%> (ø)
ldap2pg/validators.py	100.00% <100.00%> (ø)
ldap2pg/utils.py	100.00% <0.00%> (+0.63%)	⬆️

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