export: avoid error on unbound variable

Might stop sourcing export.sh when `set -u` is used (typically with
direnv):
> /path/to/esp-idf/export.sh:16: IDF_EXPORT_QUIET: unbound variable

export.sh

@@ -13,7 +13,7 @@ __realpath() {
 
 
 __verbose() {
-    [ -n "${IDF_EXPORT_QUIET}" ] && return
+    [ -n "${IDF_EXPORT_QUIET-}" ] && return
     echo "$@"
 }