Fix bug for incompressible FSI cases.

[rsanfer]

Proposed Changes

Fixes a bug introduced in #756 for incompressible FSI cases.

Related Work

This kind of bugs could be avoided with a strategy as suggested by @pcarruscag in #770

PR Checklist

Put an X by all that apply. You can fill this out after submitting the PR. If you have any questions, don't hesitate to ask! We want to help. These are a guide for you to know what the reviewers will be looking for in your contribution.

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with the '-Wall -Wextra -Wno-unused-parameter -Wno-empty-body' compiler flags).
• My contribution is commented and consistent with SU2 style.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp) , if necessary.

[rsanfer]

I don't know where this extra commit from github actions comes from. @talbring ??

[talbring]

Sorry, apparently there is a "bug" in the github actions system ... should work now.

[talbring]

I did a force push, so you have to delete your local branch and fetch the new one in case you want to modify it locally.

[rsanfer]

Tests are passing, so this wasn't covered. I will make sure it's covered via the tutorials.

[pcarruscag]

LGTM