chore(ci): fix failure condition when modified files are empty

Signed-off-by: Kotaro Yoshimoto <pythagora.yoshimoto@gmail.com>

.github/workflows/cppcheck-differential.yaml

@@ -31,21 +31,20 @@ jobs:
         run: git fetch origin ${{ github.base_ref }}
         shell: bash
 
-      - name: Get changed files
-        id: changed-files
+      - name: Get modified files
+        id: get-modified-files
         run: |
-          git diff --name-only "origin/${{ github.base_ref }}"...HEAD > changed_files.txt
-          cat changed_files.txt
+          echo "changed_files=$(git diff --name-only "origin/${{ github.base_ref }}"...HEAD | grep -E '\.(cpp|hpp)$' | tr '\n' ' ' || true)" >> $GITHUB_OUTPUT
         shell: bash
-
+        
       - name: Run Cppcheck on changed files
         continue-on-error: true
         id: cppcheck
         run: |
           files=$(cat changed_files.txt | grep -E '\.(cpp|hpp)$' || true)
           if [ -n "$files" ]; then
-            echo "Running Cppcheck on changed files: $files"
-            cppcheck --enable=all --inconclusive --check-level=exhaustive --error-exitcode=1 --suppressions-list=.cppcheck_suppressions $files 2> cppcheck-report.txt
+            echo "Running Cppcheck on changed files: ${{ steps.get-modified-files.outputs.changed_files }}"
+            cppcheck --enable=all --inconclusive --check-level=exhaustive --error-exitcode=1 --suppressions-list=.cppcheck_suppressions ${{ steps.get-modified-files.outputs.changed_files }} 2> cppcheck-report.txt
           else
             echo "No C++ files changed."
             touch cppcheck-report.txt
@@ -63,5 +62,5 @@ jobs:
           path: cppcheck-report.txt
 
       - name: Fail the job if Cppcheck failed
-        if: steps.cppcheck.outcome == 'failure'
+        if: ${{ steps.get-modified-files.outputs.changed_files != '' && steps.cppcheck.outcome == 'failure' }}
         run: exit 1