Fix for empty install path

Will default to '/' due to regular usage of that var in paths

astronaut/astronaut

@@ -221,13 +221,13 @@ fi
 # Create the directories
 if [ "$_download_only" = false ]; then
     # This variable can be set by a wrapper script in need to nuke the install dir.
-    if [ "$_nuke_dir_install" = true ]; then
+    if [ "$_nuke_dir_install" = true -a ! -z "$dir_install" ]; then
         rm -rf "$dir_install"
     fi
 
     rm -rf "$dir_build"
     mkdir -p "$dir_build"
-    mkdir -p "$dir_install"
+    [ ! -z "$dir_install" ] && mkdir -p "$dir_install"
     cd "$dir_build"
 fi