Fix warnings for undefined local variable

* Add private method primary_key_for, which more clearly shows that the
expected return value is nil when a primary key is not found.

activerecord/lib/active_record/schema_dumper.rb

@@ -105,18 +105,22 @@ HEADER
         end
       end
 
+      def primary_key_for(table)
+        if @connection.respond_to?(:pk_and_sequence_for)
+          pk, _ = @connection.pk_and_sequence_for(table)
+          return pk if pk
+        end
+        return @connection.primary_key(table) if @connection.respond_to?(:primary_key)
+        nil
+      end
+
       def table(table, stream)
         columns = @connection.columns(table)
         begin
           tbl = StringIO.new
 
           # first dump primary key column
-          if @connection.respond_to?(:pk_and_sequence_for)
-            pk, _ = @connection.pk_and_sequence_for(table)
-          end
-          if !pk && @connection.respond_to?(:primary_key)
-            pk = @connection.primary_key(table)
-          end
+          pk = primary_key_for(table)
 
           tbl.print "  create_table #{remove_prefix_and_suffix(table).inspect}"
           pkcol = columns.detect { |c| c.name == pk }