Only execute diff validate if there are redirects (#1732)

Author: cotti

.github/workflows/preview-build.yml

@@ -300,6 +300,7 @@ jobs:
       - name: 'Validate redirect rules'
         if: >
           env.MATCH == 'true'
+          && hashFiles('docs/_redirects.yml', 'docs/redirects.yml') != ''
           && github.repository == 'elastic/docs-builder'
           && (
             steps.deployment.outputs.result
@@ -314,6 +315,7 @@ jobs:
       - name: 'Validate redirect rules'
         if: >
           env.MATCH == 'true'
+          && hashFiles('docs/_redirects.yml', 'docs/redirects.yml') != ''
           && (
             github.repository != 'elastic/docs-builder'
             && (