init-pants: Only use CI config file if it exists (#13)

Avoids manipulating any custom value for input.pants-config-files.
If DEFAULT, then set PANTS_CONFIG_FILES=pants.ci.toml if pants.ci.toml exists.
Setting input.pants-config-files='' still bypasses setting the env var.

init-pants/action.yaml

@@ -19,7 +19,8 @@ inputs:
     # should be a hash generated by hashFiles()
   pants-ci-config:
     required: false
-    default: pants.ci.toml
+    default: DEFAULT
+    # defaults to pants.ci.toml if present
     # set to empty string to disable
   cache-lmdb-store:
     required: false
@@ -105,8 +106,20 @@ runs:
     - name: Tell pants to use CI config
       if: ${{ inputs.pants-ci-config != '' }}
       shell: bash
+      env:
+        PANTS_CONFIG_FILES: ${{ inputs.pants-ci-config }}
       run: |
-        echo "PANTS_CONFIG_FILES=${{ inputs.pants-ci-config }}" >> ${GITHUB_ENV}
+        if [ "${PANTS_CONFIG_FILES}" == "DEFAULT" ]; then
+          if [ -f "pants.ci.toml" ]; then
+            echo "PANTS_CONFIG_FILES=pants.ci.toml" >> ${GITHUB_ENV}
+          else
+            echo "NOT setting PANTS_CONFIG_FILES by default"
+            echo "because pants.ci.toml does not exist."
+          fi
+        else
+          echo "Setting custom PANTS_CONFIG_FILES=${PANTS_CONFIG_FILES}"
+          echo "PANTS_CONFIG_FILES=${PANTS_CONFIG_FILES}" >> ${GITHUB_ENV}
+        fi
 
     - name: Bootstrap Pants
       shell: bash