pythonPackages.bootstrapped-pip: do not assume pip has a postPatch field

This was breaking poetry2nix which passed `pip` without any `postPatch`
field, presumbly from a `poetry.lock`-induced dependency tree.

pkgs/development/python-modules/bootstrapped-pip/default.nix

@@ -24,7 +24,7 @@ stdenv.mkDerivation rec {
 
   postPatch = ''
     mkdir -p $out/bin
-  '' + pip.postPatch;
+  '' + (pip.postPatch or ""); # `pip` does not necessarily have a `postPatch` field.
 
   nativeBuildInputs = [ makeWrapper unzip ];
   buildInputs = [ python ];