remove check for present? from delete_all

Passing in a blank string is a bug so there's no reason
to check for a blank string.

activerecord/lib/active_record/associations/collection_association.rb

@@ -182,11 +182,11 @@ def transaction(*args)
       #
       # See delete for more info.
       def delete_all(dependent = nil)
-        if dependent.present? && ![:nullify, :delete_all].include?(dependent)
+        if dependent && ![:nullify, :delete_all].include?(dependent)
           raise ArgumentError, "Valid values are :nullify or :delete_all"
         end
 
-        dependent = if dependent.present?
+        dependent = if dependent
                       dependent
                     elsif options[:dependent] == :destroy
                       :delete_all