[4.x] Fix #603 - Add guard code to collections, tidy up Workflow serializer

[ricardozanini]

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Fix #603

What this PR does / why we need it:

Special notes for reviewers:
@fjtirado I've made a "breaking" change. Previously, the Workflow.toJson() methods would swallow all exceptions, logging them to the console. Since the API did not come with an SLF4J implementation, the message will not be displayed, and the exception will be lost forever.

Instead, we now throw an IllegalException to fail fast and avoid forcing client code to catch exceptions.

@gmunozfe, mind taking a look too? Perhaps we can give it a try with Kogito and assess this change impact.

Additional information (if needed):

[fjtirado]

@ricardozanini actually toJson and to toYaml behaviour was inconsistent with the fact that fromSource already throw an IllegalArgumentException ( I guess that what you mean, you did not touch fromSource, but toJson and toYaml)