Make shellcheck happier

dnsapi/dns_he.sh

@@ -20,6 +20,8 @@ dns_he_add() {
   _info "Using DNS-01 Hurricane Electric hook"
 
   if [ -z "$HE_Username" ] || [ -z "$HE_Password" ]; then
+    HE_Username=
+    HE_Password=
     _err "No auth details provided. Please set user credentials using the \$HE_Username and \$HE_Password envoronment variables."
     return 1
   fi