Fix the datetime-to-str converters

[mdickinson]

This PR fixes the datetime to-and-from string conversions used in the DatetimeEditor:

• For str-to-datetime, allow the empty string to be converted to None (a Datetime trait allows None by default)
• For str-to-datetime, we can reasonably assume that the argument is a string, so the is not None check is pointless. Remove it.
• For datetime-to-str, make sure we always return a string, returning the empty string for an input of None.

Fixes #936

[codecov-io]

Codecov Report

Merging #937 into master will increase coverage by 0.20%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #937      +/-   ##
==========================================
+ Coverage   72.90%   73.11%   +0.20%     
==========================================
  Files          51       51              
  Lines        6456     6458       +2     
  Branches     1301     1301              
==========================================
+ Hits         4707     4722      +15     
+ Misses       1357     1341      -16     
- Partials      392      395       +3     

Impacted Files	Coverage Δ
traits/editor_factories.py	79.59% <0.00%> (-4.79%)	⬇️
traits/trait_types.py	71.07% <0.00%> (-0.16%)	⬇️
traits/traits.py	76.92% <0.00%> (+4.04%)	⬆️
traits/etsconfig/etsconfig.py	63.46% <0.00%> (+6.41%)	⬆️

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[kitchoi]

LGTM