Fix USE_FASTBOOT: unbound variable error when variable is not set

Explicitly default to an empty value, to work with `set -u`.

Author: jtgeibel

script/ember.sh

@@ -3,7 +3,7 @@ set -ue
 
 export FASTBOOT_DISABLED
 
-if [ -z "${USE_FASTBOOT}" ]; then
+if [ -z "${USE_FASTBOOT-}" ]; then
     unset FASTBOOT_DISABLED
 else
     FASTBOOT_DISABLED=1

script/start-web.sh

@@ -1,7 +1,7 @@
 #! /bin/bash
 set -ue
 
-if [[ -z "${USE_FASTBOOT}" ]]; then
+if [[ -z "${USE_FASTBOOT-}" ]]; then
     unset USE_FASTBOOT
     bin/start-nginx ./target/release/server
 else