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-check-hook.sh

@@ -12,7 +12,7 @@ setuptoolsCheckPhase() {
     echo "Finished executing setuptoolsCheckPhase"
 }
 
-if [ -z "${dontUseSetuptoolsCheck-}" ] && [ -z "$installCheckPhase" ]; then
+if [ -z "${dontUseSetuptoolsCheck-}" ] && [ -z "${installCheckPhase-}" ]; then
     echo "Using setuptoolsCheckPhase"
     preDistPhases+=" setuptoolsCheckPhase"
 fi