[chore] skip stable module check if unset (#10734)

From
https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/release.md#releasing-opentelemetry-collector:

> If not intending to release stable modules, do not specify a version
for Release candidate version stable.

`validate-versions` workflow fails on empty version right now, it should
instead skip if stable module candidate version is empty.

.github/workflows/prepare-release.yml

@@ -44,7 +44,9 @@ jobs:
 
           validate_beta_version "${{ inputs.candidate-beta }}" "candidate-beta"
           validate_beta_version "${{ inputs.current-beta }}" "current-beta"
-          validate_stable_version "${{ inputs.candidate-stable }}" "candidate-stable"
+          if [[ ! -z "${{ inputs.candidate-stable }}" ]]; then
+            validate_stable_version "${{ inputs.candidate-stable }}" "candidate-stable"
+          fi
           validate_stable_version "${{ inputs.current-stable }}" "current-stable"
         shell: bash