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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• My changes generate no new warnings (e.g. Sonar is happy)
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[wernerdv]

@Haarolean Hello, PTAL.