set default serverPort when there's no applicationIndex

[Tcharl]

fixes #19782

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[Tcharl]

Oups, sorry @DanielFran , it was a code point

[mshima]

It workaround the initial generation, but others configs and generators may still be broken, see #20839

[Tcharl]

Both PRs are complementary and valid to my understanding.
The issue which is fixed by this one is an empty port entry in a spring configuration which makes configuration validation to fail: http://localhost:.

Ok, I see what you mean. Closing this one