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.

1 files changed, 1 insertions, 1 deletions

diff --git a/pkgs/development/python-modules/bootstrapped-pip/default.nix b/pkgs/development/python-modules/bootstrapped-pip/default.nix
index 812ff1a64cb..d2cefade9c4 100644
--- a/pkgs/development/python-modules/bootstrapped-pip/default.nix
+++ b/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 ];