Fix codestart properties with yaml null key

[ia3andy]

In codestarts, config is always written in yaml.

When using ~ as documented here https://quarkus.io/guides/config-yaml#configuration-key-conflicts, then generated application.properties is wrong .~=:
e.g.:

quarkus:
  quinoa:
    package-manager-install:
      ~: true
      node-version: 16.17.0
    dev-server:
      port: 3000

gives:

quarkus.quinoa.package-manager-install.~=true
quarkus.quinoa.dev-server.port=3000
quarkus.quinoa.package-manager-install.node-version=16.17.0

This fix is making sure we generate this instead:

quarkus.quinoa.package-manager-install=true
quarkus.quinoa.dev-server.port=3000
quarkus.quinoa.package-manager-install.node-version=16.17.0