Fix the exception of empty value conversion

[Lance52259]

What this PR does / why we need it:

• When using d.get ("db.0.port").(int) to get an optional parameter that is not set, a zero value is obtained.
  d.GetOK() method can be successfully to fix it.
• Two resources with this problem:
  • huaweicloud_gaussdb_cassandra_instance
  • huaweicloud_rds_instance

Which issue this PR fixes:
(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)
fixes #1204

Special notes for your reviewer:

Release note:

1. check the value whether is exist before conversion.

PR Checklist

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

Acceptance Steps Performed

make testacc TEST='./huaweicloud' TESTARGS='-run=TestAccGeminiDBInstance_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run=TestAccGeminiDBInstance_basic -timeout 360m -parallel 4
=== RUN   TestAccGeminiDBInstance_basic
=== PAUSE TestAccGeminiDBInstance_basic
=== CONT  TestAccGeminiDBInstance_basic
--- PASS: TestAccGeminiDBInstance_basic (1107.38s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       1107.425s