Don't exclude anything in test matrix if impacted modules list is empty

[nineinchnick]

Description

This is a follow up to #10323 that did not properly handled running GIB on the master branch. This PR ensures that:

• the impacted modules log file gib-impacted.log always exists
• nothing is excluded from the test matrix if the file is empty

Is this change a fix, improvement, new feature, refactoring, or other?
other - ci fix

Is this a change to the core query engine, a connector, client library, or the SPI interfaces? (be specific)
other - ci fix

How would you describe this change to a non-technical end user or system administrator?
n/a

Related issues, pull requests, and links

Documentation

(x) No documentation is needed.
( ) Sufficient documentation is included in this PR.
( ) Documentation PR is available with #prnumber.
( ) Documentation issue #issuenumber is filed, and can be handled later.

Release notes

(x) No release notes entries required.
( ) Release notes entries required with the following suggested text:

# Section
* Fix some things. ({issue}`issuenumber`)