fix a bug in cce cluster

[Jason-Zhang9309]

What this PR does / why we need it:

Fix a bug in cce cluster, which cause err when param masters is empty.

Which issue this PR fixes:
(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)
fixes #xxx

Special notes for your reviewer:

Release note:

PR Checklist

• Tests added/passed.
• Documentation updated.
• Schema updated.

Acceptance Steps Performed

make testacc TEST='./huaweicloud' TESTARGS='-run TestAccCCEClusterV3_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccCCEClusterV3_basic -timeout 360m -parallel 4
=== RUN   TestAccCCEClusterV3_basic
=== PAUSE TestAccCCEClusterV3_basic
=== CONT  TestAccCCEClusterV3_basic
--- PASS: TestAccCCEClusterV3_basic (487.18s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       487.248s