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/pip-install-hook.sh

@@ -18,7 +18,7 @@ pipInstallPhase() {
     echo "Finished executing pipInstallPhase"
 }
 
-if [ -z "${dontUsePipInstall-}" ] && [ -z "$installPhase" ]; then
+if [ -z "${dontUsePipInstall-}" ] && [ -z "${installPhase-}" ]; then
     echo "Using pipInstallPhase"
     installPhase=pipInstallPhase
 fi