Method that concatenates strings, some of which may be null, with several different separators
The repeated task here is that you want to conditionally insert a separator between two strings if both are non-null.  So, write a helper function that does that:

private static String join(String a, String sep, String b) {
    return (a == null) ? b :
           (b == null) ? a : a + sep + b;
}


Then, the code becomes simple:

private String createString(String modeloVotacao, String tipoProposicao, String ordemNumero, String ordemAno) {       
    String description = join(modeloVotacao, " - ",
                             join(tipoProposicao, " ",
                                 join(ordemNumero, "/", ordemAno)));
    return (description == null) ? "" : description;
}


Note that in your final example, where ordemNumero and ordemAno are both null, the output ends with a space.  That does not happen in my solution; I consider that to be an improvement.

Please avoid mixing Portuguese with English identifiers (such as createString()).  Stick with one or the other (preferably English).