[run-clang-tidy] Use -v to check if RUN_CLANG_TIDY is set

tools/run-clang-tidy

@@ -24,8 +24,6 @@ RUN_CLANG_TIDY_CANDIDATES=(
 	/usr/share/clang/run-clang-tidy.py
 )
 
-RUN_CLANG_TIDY=""
-
 for candidate in ${RUN_CLANG_TIDY_CANDIDATES[@]}; do
 	if ! command -v "${candidate}"; then
 		continue;
@@ -35,7 +33,7 @@ for candidate in ${RUN_CLANG_TIDY_CANDIDATES[@]}; do
 	break;
 done
 
-if [[ -z "${RUN_CLANG_TIDY}" ]]; then
+if [[ ! -v RUN_CLANG_TIDY ]]; then
 	echo "No run-clang-tidy executable found"
 	exit 1
 fi