Solve pandas FutureWarning on behaviour of array concatenation

[berland]

Observed when running tests

$ pytest -sxv tests/forward_models/design2params/test_design2params.py::test_logging_order
  /data/projects/semeio/src/semeio/forward_models/design2params/design2params.py:124: FutureWarning: The behavior of array concatenation with empty entries is deprecated. In a future version, this will no longer exclude empty items when determining the result dtype. To retain the old behavior, exclude the empty entries before the concat operation.
    merged["combined"] = merged["parameters_realization"].combine_first(

[berland]

Prioritization is now affected by deprecation of design2params in ert.