numpy DeprecationWarnings

[dweindl]

test/petab/test_amici_objective.py::test_error_leastsquares_with_ssigma
test/petab/test_amici_objective.py::test_preeq_guesses
test/petab/test_petab_import.py::PetabImportTest::test_3_optimize
  /home/runner/work/pyPESTO/pyPESTO/pypesto/problem/base.py:240: DeprecationWarning: The truth value of an empty array is ambiguous. Returning False, but in future this will result in an error. Use `array.size > 0` to check that an array is not empty.
    if self.x_guesses == np.zeros((0, self.dim_full)):

test/petab/test_petab_import.py::PetabImportTest::test_3_optimize
test/petab/test_petab_import.py::PetabImportTest::test_3_optimize
test/petab/test_petab_import.py::PetabImportTest::test_check_gradients
test/petab/test_petab_import.py::PetabImportTest::test_check_gradients
test/petab/test_petab_import.py::test_plist_mapping
test/petab/test_petab_import.py::test_plist_mapping
  /home/runner/work/pyPESTO/pyPESTO/pypesto/problem/base.py:240: DeprecationWarning: elementwise comparison failed; this will raise an error in the future.
    if self.x_guesses == np.zeros((0, self.dim_full)):