BE: Set default message polling mode

[wernerdv]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)
According to MessagesApi the mode argument (Messages polling mode) is optional, so a null value is possible.
However, the getTopicMessagesV2 method has a check for a non-null value for the mode argument.
The suggested solution is to set the default value to LATEST if mode is null.

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• I have performed a self-review of my own code
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• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

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[wernerdv]

@Haarolean Hello, PTAL.

[Haarolean]

@wernerdv thanks!