Fix the problem that any type of DNAT rule cannot open all ports

[Lance52259]

What this PR does / why we need it:
Nat dnat rule cannot create which request set with zero service port.
This issue because of isEmptyValue function return false when service port is zero(int) and program will abandom setting data.

Which issue this PR fixes:
(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)
fixes #880

Special notes for your reviewer:

Release note:

1. remove isEmptyValue function check because service port can be set with zero.

PR Checklist

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

Acceptance Steps Performed

make testacc TEST='./huaweicloud' TESTARGS='-run=TestAccNatDnat_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run=TestAccNatDnat_basic -timeout 360m -parallel 4
=== RUN   TestAccNatDnat_basic
=== PAUSE TestAccNatDnat_basic
=== CONT  TestAccNatDnat_basic
--- PASS: TestAccNatDnat_basic (167.94s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       167.984s

make testacc TEST='./huaweicloud' TESTARGS='-run=TestAccNatDnat_protocol'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run=TestAccNatDnat_protocol -timeout 360m -parallel 4
=== RUN   TestAccNatDnat_protocol
=== PAUSE TestAccNatDnat_protocol
=== CONT  TestAccNatDnat_protocol
--- PASS: TestAccNatDnat_protocol (163.30s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       163.349s