handle nil value for key when strict_variables is on

[nijikon]

Fixes #749

[dylanahsmith]

strict_variables really shouldn't be raising in lookup_and_evaluate or we should be calling something else here, since this won't work properly if @evironments has a length of more than 1.

For example:

template = Liquid::Template.parse("{{ foo }}")
template.assigns["foo"] = "works"
puts template.render!({}).inspect  # => "works"
puts template.render!({}, strict_variables: true).inspect  # raises Liquid::UndefinedVariable

[dylanahsmith]

Please add a regression test for #749 and the issue I pointed out in #750 (comment), so we can try to fix this properly this time.