SAML SLO Cert Fix

[ymylei]

If you upload an invalid cert, you can populate these other fields but the certificate field will be null. This check avoids a provider crash when this scenario appears and a refresh happens.

[ymylei]

Test results:

➜  terraform-provider-okta git:(saml-fix) TEST_FILTER=TestAccAppSaml_crud make testacc
go mod tidy
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v  -run TestAccAppSaml_crud -timeout 120m
?       github.com/okta/terraform-provider-okta [no test files]
=== RUN   TestAccAppSaml_crud
--- PASS: TestAccAppSaml_crud (51.24s)
PASS
ok      github.com/okta/terraform-provider-okta/okta    51.502s
testing: warning: no tests to run
PASS
ok      github.com/okta/terraform-provider-okta/okta/internal/apimutex  (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/okta/terraform-provider-okta/okta/internal/transport (cached) [no tests to run]
?       github.com/okta/terraform-provider-okta/sdk     [no test files]

[monde]

@ymylei I'm going to cherry-pick d055788 into our v3.20.4 release as I don't want to bring in the go mod changes. Thanks for the fix.