Fix Time fields bug which occurs when Time value is nil

[rnitta]

When there are null-allowed Time column on DB,

administrate/app/views/fields/time/_index.html.erb

Line 17 in 8d86f9f

<%= field.data.strftime("%I:%M%p").to_s %>

This code can raise error, becauseundefined method 'strftime' for nil:NilClass if field.data is nil

so I add code to check if time value is nill and some specs

please check it.

and,
CircleCi tests failed due to dependencies vulnerability.
please check this, too. #1225

[GeoffAbtion]

Is it possible to rerun the CI on this PR, the dependencies should be up to date now.

[nickcharlton]

Thanks for your work here! I rebased and now I'll merge it.