TBR: mount_gpt_image: fix missing guard for esp

Missing check during mkdir causes failure with -e empty.

The tree broke prior to this commit, but this will break the dev
build so bypassing hooks. sorry.

TEST=reran build_image
BUG=none
TBR=adlr

mount_gpt_image.sh

@@ -88,6 +88,7 @@ function get_gpt_partitions() {
 function mount_image() {
   mkdir -p "${FLAGS_rootfs_mountpt}"
   mkdir -p "${FLAGS_stateful_mountpt}"
+  test -n "${FLAGS_esp_mountpt}" && \
     mkdir -p "${FLAGS_esp_mountpt}"
 
   # Get the partitions for the image / device.