[incubator-kie-issues#1775] Springboot apps - Conditionally overwrite code-related context properties only if twin system properties are not null

[gitgabrio]

Fixes apache/incubator-kie-issues#1775

This PR:

1. overwrite the code-gen context properties only if related system properties are not null/empty
2. unit test newly added methods/checks

How to replicate CI configuration locally?

Build Chain tool does "simple" maven build(s), the builds are just Maven commands, but because the repositories relates and depends on each other and any change in API or class method could affect several of those repositories there is a need to use build-chain tool to handle cross repository builds and be sure that we always use latest version of the code for each repository.

build-chain tool is a build tool which can be used on command line locally or in Github Actions workflow(s), in case you need to change multiple repositories and send multiple dependent pull requests related with a change you can easily reproduce the same build by executing it on Github hosted environment or locally in your development environment. See local execution details to get more information about it.

[yesamer]

@gitgabrio Thank you for the fix and adding the Unit Tests. So, with this change, we are now saying the code-gen MUST be explicitly enabled, otherwise is disabled by default. Is this change consistent with all the examples/documentation we have?

[gitgabrio]

@yesamer
code-gen is already enabled by default (see here). This fix allows user to disable it on demand.