fix(usemin): do not clear generated file if section not found

Before this change, configuring a usemin section that does not exist caused the concat/generated/files section to be set to `false`. We now leave it unchanged in case the `appender.save()` is invoked with `false`. I didn't write a test for this, I just added a config with a "not found" section to `grunt` and the fact that the existing `usemin` test works correctly means that it is working...