txnbuild: CreateClaimableBalance.Validate() accepts empty Destinations #6007

Bug

CreateClaimableBalance.Validate() returns nil when Destinations is nil or an empty slice. stellar-core rejects such operations with CREATE_CLAIMABLE_BALANCE_MALFORMED (the doCheckValid guard checks claimants.empty()), so the SDK should catch this at build time.

Reproduce

op := &txnbuild.CreateClaimableBalance{
    Amount: "10.0000000",
    Asset:  txnbuild.NativeAsset{},
    // Destinations intentionally omitted
}
err := op.Validate()
fmt.Println(err) // <nil>  -- should be a validation error

Root cause

The for _, d := range cb.Destinations loop in Validate() is a no-op when the slice is nil or empty, so the method falls through to the amount/asset checks and returns nil.

Note

js-stellar-base already validates this:

if (!Array.isArray(opts.claimants) || opts.claimants.length === 0) {
  throw new Error('must provide at least one claimant');
}