get_loaded_model_by_path is supposed to be nil-safe

[sato11]

This commit partially reverts #801, which by declaring conditional return
has turned get_loaded_model_by_path to a less safe method that can
return nil when its condition is not met.

Apparently the very same condition has been brought to annotate_model_file
by #774, which seems to cover the "bug" insisted in #801 as well.

On the other hand #801 has brought an inconvenient behaviour as well:
whenever a non-activerecord model file is found, get_loaded_model_by_path
returns nil, which leads to raising BadModelFileError and ends up
printing a bunch of "Unable to annotate ..." messages.

Now it seems tests added by #801 are running right and I do not find
a problem restoring the previous behaviour and turn it nil-safe again.

---

Say we have a file like app/models/foo/bar.rb which is NOT a subclass of ActiveRecord::Base:

class Foo::Bar
end

We do not want this to get annotated but the current code does give it a try, ending up showing a bunch of error messages like this:

Unable to annotate app/models/foo/bar.rb: file doesn't contain a valid model class
Unable to annotate app/models/foo/baz.rb: file doesn't contain a valid model class
...

With this change the behaviour in v3.1.1 is restored and these un-informational errors are gone.

[sato11]

@ctran
Will you take a look at this before proceeding with v3.1.2? (#881)
The CI's failed but I guess it's a config issue since all the PRs have failed too since mid-April?

[ctran]

I'll take a look at the CI configuration. You want this to be merged for 3.1.2 right?

[sato11]

Yes I do since v3.1.2 would introduce breaking change if shipped without this. Thanks for the quick action! 🙌