Fix get_time_index bug

[eccabay]

We were doing some unnecessary conversion of a DatetimeIndex into a Series and then back, which threw some errors with certain datetime data. This fixes both those issues!

[codecov]

Codecov Report

Merging #3792 (d6213df) into main (b5eb7b0) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3792     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        343     343             
  Lines      35797   35813     +16     
=======================================
+ Hits       35660   35676     +16     
  Misses       137     137             

Impacted Files	Coverage Δ
evalml/tests/utils_tests/test_gen_utils.py	99.5% <100.0%> (+0.1%)	⬆️
evalml/utils/gen_utils.py	99.3% <100.0%> (+0.1%)	⬆️

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