chore(verify): Default to public key verification

verify/action.yml

@@ -15,7 +15,8 @@ inputs:
     description: 'The certificate OIDC issuer'
     required: false
   verification:
-    description: 'The type of verification to use (certificate or public-key)'
+    description: 'The type of verification to use, certificate or publickey'
+    default: 'public-key'
     required: true
 runs:
   using: "composite"
@@ -36,6 +37,9 @@ runs:
         if [[ -z ${{ inputs.container }} ]]; then
           exit 1
         fi
+        if [[ -z ${{ inputs.verification }} ]]; then
+          exit 1
+        fi
         if [[ ${{ inputs.verification }} == "public-key" ]]; then
           if [[ -n ${{ inputs.pubkey }} ]]; then
             cosign verify --key ${{ inputs.pubkey }} ${{ steps.container_case.outputs.lowercase }}