stdenv: Fix possible issues discovered with

```
set -o errexit -o nounset -o pipefail
shopt -s inherit_errexit
```

in `stdenv/default-builder.sh`

pkgs/build-support/setup-hooks/multiple-outputs.sh

···
     # try to detect share/doc/${shareDocName}
     # Note: sadly, $configureScript detection comes later in configurePhase,
     #   and reordering would cause more trouble than worth.
-    if [ -z "$shareDocName" ]; then
-        local confScript="$configureScript"
+    if [ -z "${shareDocName:-}" ]; then
+        local confScript="${configureScript:-}"
         if [ -z "$confScript" ] && [ -x ./configure ]; then
             confScript=./configure
         fi

pkgs/build-support/setup-hooks/patch-shebangs.sh

···
         read -r oldInterpreterLine < "$f"
         read -r oldPath arg0 args <<< "${oldInterpreterLine:2}"
 
-        if [[ -z "$pathName" ]]; then
+        if [[ -z "${pathName:-}" ]]; then
             if [[ -n $strictDeps && $f == "$NIX_STORE"* ]]; then
                 pathName=HOST_PATH
             else

pkgs/stdenv/generic/setup.sh

···
     done
 
     sed -i "$1" \
-        -e "s^eval \(sys_lib_search_path=\).*^\1'$search_path'^" \
+        -e "s^eval \(sys_lib_search_path=\).*^\1'${search_path:-}'^" \
         -e 's^eval sys_lib_.+search_path=.*^^'
 }