FIX: prevents exception when text input is nil (PR #12922)

nil was converted to “” and the matching regex would return and then be converted to nil with max usage.

Example exception:

NoMethodError (undefined method `<=' for nil:NilClass)

lib/text_sentinel.rb:71:in `seems_unpretentious?'
lib/validators/quality_title_validator.rb:13:in `validate_each'
lib/topic_creator.rb:25:in `valid?'

GitHub