fix: Quote fprettify output

Author: JamieJQuinn

.github/workflows/check_policies.yml

@@ -19,6 +19,6 @@ jobs:
       - uses: actions/checkout@v3
       - run: pip install fprettify
       - run: fprettify --config .fprettify.ini --diff --recursive src
-      - run: if [ ! -z $($FPRETTIFY_COMMAND) ]; then echo "::warning::Code formatting issues detected"; fi
+      - run: if [[ ! -z "$($FPRETTIFY_COMMAND)" ]]; then echo "::warning::Code formatting issues detected"; fi
         env:
           FPRETTIFY_COMMAND: fprettify --config .fprettify.ini --diff --recursive src