Fix inconsistency between SDK and framework by nil relaxing

[tosuke]

Output from acceptance testing:

$ make testacc TESTS='AccCompat'
TF_ACC=1 go test -v ./... -run AccCompat -timeout 120m
?       github.com/mackerelio-labs/terraform-provider-mackerel  [no test files]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/mackerel        0.702s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/planmodifierutil        0.516s [no tests to run]
=== RUN   TestAccCompat_MackerelAlertGroupSettingResource
=== PAUSE TestAccCompat_MackerelAlertGroupSettingResource
=== RUN   TestAccCompat_MackerelAWSIntegrationResource
=== PAUSE TestAccCompat_MackerelAWSIntegrationResource
=== RUN   TestAccCompat_MackerelChannelResource_Email
=== PAUSE TestAccCompat_MackerelChannelResource_Email
=== RUN   TestAccCompat_MackerelChannelResource_Slack
=== PAUSE TestAccCompat_MackerelChannelResource_Slack
=== RUN   TestAccCompat_MackerelChannelResource_Webhook
=== PAUSE TestAccCompat_MackerelChannelResource_Webhook
=== RUN   TestAccCompat_MackerelDowntimeResource
=== PAUSE TestAccCompat_MackerelDowntimeResource
=== RUN   TestAccCompat_MackerelNotificationGroupResource
=== PAUSE TestAccCompat_MackerelNotificationGroupResource
=== CONT  TestAccCompat_MackerelAlertGroupSettingResource
=== CONT  TestAccCompat_MackerelChannelResource_Webhook
=== RUN   TestAccCompat_MackerelAlertGroupSettingResource/minimal
=== PAUSE TestAccCompat_MackerelAlertGroupSettingResource/minimal
=== RUN   TestAccCompat_MackerelAlertGroupSettingResource/empty
=== PAUSE TestAccCompat_MackerelAlertGroupSettingResource/empty
=== CONT  TestAccCompat_MackerelAlertGroupSettingResource/minimal
=== CONT  TestAccCompat_MackerelChannelResource_Email
=== CONT  TestAccCompat_MackerelNotificationGroupResource
=== CONT  TestAccCompat_MackerelDowntimeResource
=== RUN   TestAccCompat_MackerelDowntimeResource/null
=== PAUSE TestAccCompat_MackerelDowntimeResource/null
=== RUN   TestAccCompat_MackerelDowntimeResource/empty
=== PAUSE TestAccCompat_MackerelDowntimeResource/empty
=== RUN   TestAccCompat_MackerelDowntimeResource/undefined
=== PAUSE TestAccCompat_MackerelDowntimeResource/undefined
=== CONT  TestAccCompat_MackerelDowntimeResource/null
=== CONT  TestAccCompat_MackerelChannelResource_Slack
=== CONT  TestAccCompat_MackerelDowntimeResource/undefined
=== CONT  TestAccCompat_MackerelDowntimeResource/empty
=== CONT  TestAccCompat_MackerelAWSIntegrationResource
=== RUN   TestAccCompat_MackerelAWSIntegrationResource/basic
=== CONT  TestAccCompat_MackerelAlertGroupSettingResource/empty
=== PAUSE TestAccCompat_MackerelAWSIntegrationResource/basic
=== CONT  TestAccCompat_MackerelAWSIntegrationResource/basic
--- PASS: TestAccCompat_MackerelAlertGroupSettingResource (0.00s)
    --- PASS: TestAccCompat_MackerelAlertGroupSettingResource/minimal (13.45s)
    --- PASS: TestAccCompat_MackerelAlertGroupSettingResource/empty (13.51s)
--- PASS: TestAccCompat_MackerelDowntimeResource (0.00s)
    --- PASS: TestAccCompat_MackerelDowntimeResource/empty (13.64s)
    --- PASS: TestAccCompat_MackerelDowntimeResource/null (13.65s)
    --- PASS: TestAccCompat_MackerelDowntimeResource/undefined (13.75s)
--- PASS: TestAccCompat_MackerelChannelResource_Webhook (13.88s)
--- PASS: TestAccCompat_MackerelChannelResource_Slack (13.93s)
--- PASS: TestAccCompat_MackerelNotificationGroupResource (13.95s)
--- PASS: TestAccCompat_MackerelChannelResource_Email (14.02s)
--- PASS: TestAccCompat_MackerelAWSIntegrationResource (0.00s)
    --- PASS: TestAccCompat_MackerelAWSIntegrationResource/basic (14.38s)
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/provider        15.307s
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/typeutil        0.380s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/validatorutil   0.827s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/mackerel 1.363s [no tests to run]