simplify by using -n. SC2236

I agree to license my contributions to each file under the terms given
at the top of each file I changed.

mk/check-symbol-prefixes.sh

@@ -47,7 +47,7 @@ nm_exe="${llvm_root}/llvm-nm"
 find "target/$target" -type f -name libring-*.rlib | while read -r infile; do
   bad=$($nm_exe --defined-only --extern-only --print-file-name "$infile" \
     | ( grep -v -E " . _?(__imp__ZN4ring|ring_core_|__rustc|_ZN|DW.ref.rust_eh_personality)" || [[ $? == 1 ]] ))
-  if [ ! -z "${bad-}" ]; then
+  if [ -n "${bad-}" ]; then
     echo "$bad"
     exit 1
   fi