fix: support for missing files in the results or results directory

[MaineK00n]

What did you implement:

In the initial state, there are no results under the integration directory, and make diff will generate an error. Therefore, we need to add directory and file checks.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

I deleted the integration/results file and ran make diff, and confirmed that it succeeded.

$ rm -rf integration/results/
$ make diff

Checklist:

You don't have to satisfy all of the following.

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES

Reference

• https://vuls.io/docs/en/testing.html

[kotakanbe]

Add here as well

https://github.com/future-architect/vuls/blob/master/GNUmakefile#L125
https://github.com/future-architect/vuls/blob/master/GNUmakefile#L138

mistake

[kotakanbe]

Many Thanks :)