don't add arel or uses if there is no arel or uses

This simplifies the method calls

Author: kbrock

lib/active_record/virtual_attributes.rb

@@ -103,8 +103,8 @@ def load_schema!
 
       def define_virtual_attribute(name, cast_type, uses: nil, arel: nil)
         attribute_types[name.to_s] = cast_type
-        define_virtual_include(name, uses) if uses
-        define_virtual_arel(name, arel) if arel
+        define_virtual_include(name, uses)
+        define_virtual_arel(name, arel)
       end
     end
   end

lib/active_record/virtual_attributes/virtual_arel.rb

@@ -111,7 +111,7 @@ def arel_for_virtual_attribute(column_name, table) # :nodoc:
         private
 
         def define_virtual_arel(name, arel) # :nodoc:
-          self._virtual_arel = _virtual_arel.merge(name.to_s => arel)
+          self._virtual_arel = _virtual_arel.merge(name.to_s => arel) unless arel.nil?
         end
       end
     end

lib/active_record/virtual_attributes/virtual_includes.rb

@@ -25,7 +25,7 @@ def virtual_includes(name)
         private
 
         def define_virtual_include(name, uses)
-          self._virtual_includes = _virtual_includes.merge(name.to_s => uses)
+          self._virtual_includes = _virtual_includes.merge(name.to_s => uses) unless uses.nil?
         end
       end
     end