[Fix] fix update when source_port not set

[anton-sidelnikov]

Summary of the Pull Request

PR Checklist

• Refers to: opentelekomcloud_fw_rule_v2 source_port isn't allowed 'empty' #2711
• Tests added/passed.
• Documentation updated.
• Schema updated.
• Release notes added.

Acceptance Steps Performed

=== RUN   TestAccFWRuleV2_basic
--- PASS: TestAccFWRuleV2_basic (57.94s)
=== RUN   TestAccFWRuleV2_anyProtocol
--- PASS: TestAccFWRuleV2_anyProtocol (46.55s)
=== RUN   TestAccFWRuleV2_TCPtoICMP
--- PASS: TestAccFWRuleV2_TCPtoICMP (76.22s)
=== RUN   TestAccFWRuleV2_TCPUpdate
--- PASS: TestAccFWRuleV2_TCPUpdate (40.18s)
=== RUN   TestAccFWRuleV2_emptySourcePort
--- PASS: TestAccFWRuleV2_emptySourcePort (88.37s)
PASS

Process finished with the exit code 0

[otc-zuul]

Build succeeded.
https://zuul.otc-service.com/t/eco/buildset/117dffa40938472c87a212e805a8a17f

✔️ build-otc-releasenotes SUCCESS in 4m 07s
✔️ noop SUCCESS in 0s
✔️ otc-golangci-lint SUCCESS in 3m 04s
✔️ golang-make-test SUCCESS in 3m 49s
✔️ golang-make-vet SUCCESS in 2m 43s
✔️ tflint SUCCESS in 1m 50s
✔️ goreleaser-build SUCCESS in 7m 41s