Distraught Man at Police Station "This is fraudulent, the painting was sold to me from a gallery in Krakow as a genuine Picasso, but it's clearly a forgery. Can't you see it's signed Pykaco!"
Policeman replies "I couldn't possibly say sir because, I AM A POLICEMAN NOT AN ARTIST!"
Stating your occupation is done with the usage of Instrumental ("Jestem artystą".)
The rule of negation is NOT "Negation = Genitive". It is "Negated Accusative = Genitive". Accusative is probably the most common case, so then the learners take this rule too far. But any other case, even when negated, just stays the same. Therefore if you negate "Jestem artystą" you just arrive at "Nie jestem artystą".
In the US the police identify themselves as say I am Officer Smith." Or if there is a reference to them on the news, The announcer would say, "4 police officers were involved in a shooting...." Gender may be mentioned. Sometime it is obvious.... Officer Jennifer Whatever shot.... Just curious. Also it is a question of the proper salutation.
<3 for "Officer Jennifer Whatever" :D
I never had an occasion to address a policeman, but when an English speaker would address a policeman "Officer", a Polish one could say "panie władzo" ("władza" = power, authority). But I just googled it and it seems that it's not really used nowadays (https://sjp.pwn.pl/poradnia/haslo/Panie-wladzo;10145.html) and policemen would prefer to just be addressed the same way as you would address any adult stranger, so basically by using Formal You.
As you mentioned the oficer policji already: Does there also exist a rank like komizar policji (just trying to imagine how the German word would be translated)? Normally, I would have considered the Oficer to be a Kommissar in German, although I also do not know how many ranks exist in the police forces. But does a term like this exist, a Komizar Policji?