connect: if the token given to the vault provider returns no data avoid a panic (#9806)

Improves #9800

.changelog/9806.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+connect: if the token given to the vault provider returns no data avoid a panic
+```

agent/connect/ca/provider_vault.go

@@ -82,6 +82,8 @@ func (v *VaultProvider) Configure(cfg ProviderConfig) error {
 	secret, err := client.Auth().Token().LookupSelf()
 	if err != nil {
 		return err
+	} else if secret == nil {
+		return fmt.Errorf("Could not look up Vault provider token: not found")
 	}
 	var token struct {
 		Renewable bool