Use -n instead of ! -z in shell scripts (#4404)

Signed-off-by: Joakim Roubert <joakim.roubert@axis.com>

bin/_test-run.sh

@@ -136,7 +136,7 @@ check_if_k8s_reachable(){
 
 remove_l5d_if_exists() {
   resources=$(kubectl --context=$k8s_context get all,clusterrole,clusterrolebinding,mutatingwebhookconfigurations,validatingwebhookconfigurations,psp,crd -l linkerd.io/control-plane-ns --all-namespaces -oname)
-  if [ ! -z "$resources" ]; then
+  if [ -n "$resources" ]; then
     printf 'Removing existing l5d installation...'
     cleanup
     printf '[ok]\n'

bin/fmt

@@ -10,7 +10,7 @@ dirs=$(go list -f \{\{.Dir\}\} ./... | grep -v controller/gen)
 # dedup them.
 files=$(bin/goimports -l $dirs | sort | uniq)
 
-if [ ! -z "$files" ]; then
+if [ -n "$files" ]; then
   for file in $files; do
     bin/goimports -d "$@" "$file"
   done