(1)I don't have any pear in order to say (2)"I don't have a pear is more English. Also in case the phrase(1) in Czech would have another proper translation. Language is something living, it is not: "A" is connected to "a" , "B" is "b" eccetera. It more shaded. It is like in Dutch, you wouldn't say Ik heb "niet een" (not a) peer but you do say Ik heb geen (enkele) -not any- peer. All this for saying that my answer I don't have any pear*does not mean that you are emphasing something at all costs. It should be accepted.
Another consideration. "I don't have a pear!" means indeed "I don't have a pear [ but I do have an APPLE! ] . Here we emphasize. And thus, this interpretation would send "hrušku" at the end of the sentence. And it is not! So , "I don't have any pear" is the correct sentence ... Yes, I'm splitting hairs. But in English you say: (1).I don't have any pear. --- contrasting it with :(2).I don't have pearS.. Sentence (1) Hrušku nemám. (2) becomes Hrušky nemám. At the same way I don't HAVE a PEAR/pears! (but an apple/apples) > Nemám hrušku/hrušky ! [ ale jablko/ jablka! ]. Again, I'm sorry for splitting hairs. I have these moments (not always!).