[Action] Change how to detect code change

The method of getting the diff of git is failing now.
Changed to check code diff by filtering the result.

Signed-off-by: Gichan Jang <gichan2.jang@samsung.com>

.github/actions/gitpush/action.yml

@@ -25,8 +25,7 @@ runs:
         pushd nnstreamer.github.io
         mkdir -p ${{ inputs.dest }}
         cp -r ${{ inputs.source }} ${{ inputs.dest }}
-        git diff -s --exit-code
-        if [ $? -ne 0 ]; then
+        if git diff --shortstat | grep -q changed; then
           git config user.email "nnsuite@samsung.com"
           git config user.name "nnsuite"
           git add *