Fix handling of SSL certificates in external load balancer modules

[rodriguezsergio]

An empty ssl_certificates list will conflict with a user-defined certificate_manager_certificates value, so exclude it.

This was fixed in #2764, but only for the net-lb-app-int module and not for the net-lb-app-ext or net-lb-app-ext-regional modules.

I'm not sure why net-lb-app-int-cross-region doesn't allow a user to set a value for ssl_certificates, but I'm going to leave it alone.

Screenshot from #2760

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

[rodriguezsergio]

Test output

Running tests for net-lb-app-ext-regional

================================================================================= test session starts =================================================================================
platform darwin -- Python 3.12.5, pytest-8.3.4, pluggy-1.5.0
rootdir: /Users/srodriguez/code/github/googlecloudplatform/cloud-foundation-fabric
plugins: xdist-3.6.1
collected 1015 items / 1000 deselected / 15 selected

tests/examples/test_plan.py ...............                                                                                                                                     [100%]

=================================================================== 15 passed, 1000 deselected in 275.37s (0:04:35) ===================================================================
Running tests for net-lb-app-ext

================================================================================= test session starts =================================================================================
platform darwin -- Python 3.12.5, pytest-8.3.4, pluggy-1.5.0
rootdir: /Users/srodriguez/code/github/googlecloudplatform/cloud-foundation-fabric
plugins: xdist-3.6.1
collected 1015 items / 994 deselected / 21 selected

tests/examples/test_plan.py .....................                                                                                                                               [100%]

=================================================================== 21 passed, 994 deselected in 460.19s (0:07:40) ====================================================================
Running tests for net-lb-app-int

================================================================================= test session starts =================================================================================
platform darwin -- Python 3.12.5, pytest-8.3.4, pluggy-1.5.0
rootdir: /Users/srodriguez/code/github/googlecloudplatform/cloud-foundation-fabric
plugins: xdist-3.6.1
collected 1015 items / 999 deselected / 16 selected

tests/examples/test_plan.py ................                                                                                                                                    [100%]

=================================================================== 16 passed, 999 deselected in 557.24s (0:09:17) ====================================================================

[ludoo]

Thanks for this, and the detailed explanation!