treewide: *Phase(s)? variables are optional

If these aren't defined, the stdenv defaults are used in the `*Phase`
case, or no extra phases are done, in the `*Phases` case.

pkgs/development/interpreters/python/hooks/setuptools-build-hook.sh

@@ -36,12 +36,12 @@ setuptoolsShellHook() {
     echo "Finished executing setuptoolsShellHook"
 }
 
-if [ -z "${dontUseSetuptoolsBuild-}" ] && [ -z "$buildPhase" ]; then
+if [ -z "${dontUseSetuptoolsBuild-}" ] && [ -z "${buildPhase-}" ]; then
     echo "Using setuptoolsBuildPhase"
     buildPhase=setuptoolsBuildPhase
 fi
 
-if [ -z "${dontUseSetuptoolsShellHook-}" ] && [ -z "$shellHook" ]; then
+if [ -z "${dontUseSetuptoolsShellHook-}" ] && [ -z "${shellHook-}" ]; then
     echo "Using setuptoolsShellHook"
     shellHook=setuptoolsShellHook
 fi