Length validation handles correctly nil. Fix #7180

When nil or empty string are not allowed, they are not valid.

Conflicts:
	activemodel/CHANGELOG.md
	activemodel/lib/active_model/validations/length.rb
	activemodel/test/cases/validations/length_validation_test.rb
Merge pull request #20 from square/fix-validates-length-of-without-min

Fix validates_length_of