Handle optionality for ProblemDetails presence and value check

The ProblemDetails data is set to be optional in some cases (see e.g. ETSI GS MEC 012, Table 7.3.3.1-2).

Currently the test suite requires the ProblemDetails data to always be present.

Expected behaviour

Allow the response to contain string or empty body, as well as the ProblemDetails data.

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