codeql: Only purge python cache if CODEQL_PYTHON is set (#2888)

The environment variable will point to the python installation
that installed dependencies if dependencies were installed.[0]

[0] https://docs.github.com/en/code-security/code-scanning/creating-an-advanced-setup-for-code-scanning/customizing-your-advanced-setup-for-code-scanning?learn=code_security_actions&learnProduct=code-security#analyzing-python-dependencies

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>

.github/workflows/codeql.yml

@@ -46,8 +46,11 @@ jobs:
       - name: Autobuild
         uses: github/codeql-action/autobuild@v3
 
-      - name: Cache cleanup
+      - name: Pip cache cleanup
         shell: bash
+        # CODEQL_PYTHON is only defined if dependencies were installed [0]
+        # [0] https://docs.github.com/en/code-security/code-scanning/creating-an-advanced-setup-for-code-scanning/customizing-your-advanced-setup-for-code-scanning?learn=code_security_actions&learnProduct=code-security#analyzing-python-dependencies
+        if: ${{ env.CODEQL_PYTHON != '' }}
         run: |
           $CODEQL_PYTHON -m pip cache info
           $CODEQL_PYTHON -m pip cache purge