setup-hooks: do not pass missing dirs to find (close #15405)

find fails when called with an inexistent search path.
That situation may arise when the output is created after by a postFixup hook.
vcunat amended the PR by clarifying one more `return` to `return 0`.

pkgs/build-support/setup-hooks/separate-debug-info.sh

@@ -6,8 +6,10 @@ dontStrip=1
 fixupOutputHooks+=(_separateDebugInfo)
 
 _separateDebugInfo() {
+    [ -e "$prefix" ] || return 0
+
     local dst="${debug:-$out}"
-    if [ "$prefix" = "$dst" ]; then return; fi
+    if [ "$prefix" = "$dst" ]; then return 0; fi
 
     dst="$dst/lib/debug/.build-id"
 

pkgs/development/tools/misc/patchelf/setup-hook.sh

@@ -6,6 +6,8 @@ fixupOutputHooks+=('if [ -z "$dontPatchELF" ]; then patchELF "$prefix"; fi')
 
 patchELF() {
     local dir="$1"
+    [ -e "$dir" ] || return 0
+
     header "shrinking RPATHs of ELF executables and libraries in $dir"
 
     local i