lint: remove unnecessary negation

Also use double-brackets where possible.

Fixes SC2236

plugins/common/functions

@@ -168,7 +168,7 @@ dokku_container_log_verbose_quiet() {
 is_valid_app_name() {
   declare desc="verify app name format"
   local APP="$1"
-  [[ ! -n "$APP" ]] && dokku_log_fail "APP must not be null"
+  [[ -z "$APP" ]] && dokku_log_fail "APP must not be null"
   if [[ "$APP" =~ ^[a-z].* ]] || [[ "$APP" =~ ^[0-9].* ]]; then
     if [[ ! $APP =~ [A-Z] ]] && [[ ! $APP =~ [:] ]]; then
       return 0

plugins/docker-options/docker-args-deploy

@@ -32,7 +32,7 @@ docker_args() {
       local line
       read -r line || local DONE=true
 
-      [[ ! -n "$line" ]] && continue
+      [[ -z "$line" ]] && continue
 
       # shellcheck disable=SC1001
       case "$line" in

plugins/domains/functions

@@ -248,7 +248,7 @@ get_default_vhosts() {
       if ! ([[ "$VHOST" =~ $RE_IPV4 ]] || [[ "$VHOST" =~ $RE_IPV6 ]]); then
         local SUBDOMAIN=${APP/%\.${VHOST}/}
         local hostname=$(: | plugn trigger nginx-hostname "$APP" "$SUBDOMAIN" "$VHOST")
-        if [[ ! -n $hostname ]]; then
+        if [[ -z $hostname ]]; then
           if [[ "$APP" == *.* ]] && [[ "$SUBDOMAIN" == "$APP" ]]; then
             local hostname="${APP/\//-}"
           else

plugins/enter/subcommands/default

@@ -25,7 +25,7 @@ enter_default_cmd() {
 
   if [[ "$3" == "--container-id" ]]; then
     local DOKKU_APP_CIDS=($(get_app_container_ids "$APP"))
-    if [[ ! -n "$4" ]]; then
+    if [[ -z "$4" ]]; then
       dokku_log_warn "No container id specified."
       dokku_log_fail "Available ids for app ($APP): ${DOKKU_APP_CIDS[*]}"
     fi
@@ -38,7 +38,7 @@ enter_default_cmd() {
   else
     local DOKKU_APP_CIDS=($(get_app_container_ids "$APP" "$CONTAINER_TYPE"))
     local ID=${DOKKU_APP_CIDS[0]}
-    if [[ ! -n $ID ]]; then
+    if [[ -z $ID ]]; then
       dokku_log_warn "No containers found for type '$CONTAINER_TYPE'"
       dokku_log_fail "Available types for app ($APP): ${AVAILABLE_CONTAINER_TYPES[*]}"
     fi

plugins/scheduler-docker-local/scheduler-run

@@ -46,7 +46,7 @@ scheduler-docker-local-scheduler-run() {
   PROC_CMD=$(get_cmd_from_procfile "$APP" "$POTENTIAL_PROCFILE_KEY" || echo '')
   remove_procfile "$APP"
 
-  if [ ! -z "$PROC_CMD" ]; then
+  if [[ -n "$PROC_CMD" ]]; then
     dokku_log_info1 "Found '$POTENTIAL_PROCFILE_KEY' in Procfile, running that command"
     set -- "$PROC_CMD" "${@:2}"
   fi