backend/oss: Fixes the nil pointer panic error when missing access key or secret key

[xiaozhu36]

The PR aims to fix a invoking DescribeEndpoints panic error from missing required parameter access key or secret key.

TF_ACC=1 go test -v ./internal/backend/remote-state/oss/ -run=TestBackend -timeout 120m
=== RUN   TestBackend_impl
--- PASS: TestBackend_impl (0.00s)
=== RUN   TestBackendConfig
    backend_test.go:48: TestBackendConfig on *oss.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("terraform-backend-oss-test"), "key":cty.StringVal("first.tfstate"), "prefix":cty.StringVal("mystate"), "region":cty.StringVal("cn-beijing"), "tablestore_endpoint":cty.StringVal("https://terraformstate.cn-beijing.ots.aliyuncs.com"), "tablestore_table":cty.StringVal("TableStore")}}
--- PASS: TestBackendConfig (0.34s)
=== RUN   TestBackendConfigWorkSpace
    backend_test.go:83: TestBackendConfig on *oss.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("terraform-backend-oss-test-81"), "key":cty.StringVal("first.tfstate"), "prefix":cty.StringVal("mystate"), "region":cty.StringVal("cn-beijing"), "tablestore_endpoint":cty.StringVal("https://terraformstate.cn-beijing.ots.aliyuncs.com"), "tablestore_table":cty.StringVal("TableStore")}}
--- PASS: TestBackendConfigWorkSpace (1.79s)
=== RUN   TestBackendConfigProfile
    backend_test.go:122: TestBackendConfig on *oss.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("terraform-backend-oss-test"), "key":cty.StringVal("first.tfstate"), "prefix":cty.StringVal("mystate"), "profile":cty.StringVal("default"), "region":cty.StringVal("cn-beijing"), "tablestore_endpoint":cty.StringVal("https://terraformstate.cn-beijing.ots.aliyuncs.com"), "tablestore_table":cty.StringVal("TableStore")}}
--- PASS: TestBackendConfigProfile (0.16s)
=== RUN   TestBackendConfig_invalidKey
--- PASS: TestBackendConfig_invalidKey (0.00s)
=== RUN   TestBackend
    backend_test.go:168: TestBackendConfig on *oss.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("terraform-remote-oss-test-616fce6f"), "prefix":cty.StringVal("multi/level/path/")}}
    backend_test.go:173: TestBackendConfig on *oss.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("terraform-remote-oss-test-616fce6f"), "prefix":cty.StringVal("multi/level/path/")}}
    testing.go:299: TestBackend: testing state locking for *oss.Backend
    testing.go:299: TestBackend: *oss.Backend: empty string returned for lock, assuming disabled
    testing.go:299: TestBackend: testing state locking for *oss.Backend
    testing.go:299: TestBackend: *oss.Backend: empty string returned for lock, assuming disabled
--- PASS: TestBackend (2.09s)
PASS
ok      github.com/hashicorp/terraform/internal/backend/remote-state/oss        4.952s

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