splunk: fix STIX timestamp processing

[pcoccoli]

Fixes #855

[codecov]

Codecov Report

Merging #1084 (bc31a60) into develop (99fcd99) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1084      +/-   ##
===========================================
+ Coverage    64.72%   64.74%   +0.01%     
===========================================
  Files          519      519              
  Lines        48870    48879       +9     
===========================================
+ Hits         31633    31645      +12     
+ Misses       17237    17234       -3     

Impacted Files	Coverage Δ
...dules/splunk/stix_translation/query_constructor.py	72.67% <100.00%> (+1.08%)	⬆️
.../tests/stix_translation/test_splunk_stix_to_spl.py	83.98% <100.00%> (+1.44%)	⬆️

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