Fix reading auth0_branding when enable_custom_domain_in_emails is true

[nekootoko3]

Proposed Changes

• Enables you to set auth0_branding when custom domain is enabled and universal_login is not set
  • By current provider behavior, when custom domain is enabled, custom universal login pages must be set. From document, when custom domain is enabled, it is true that universal login pages can be set, but you don't have to.

Fixes #380

Acceptance Test Output

$ make testacc TESTS=TestAccBranding                                                         
==> Checking that code complies with gofmt requirements...
?       github.com/alexkappa/terraform-provider-auth0   [no test files]
=== RUN   TestAccBranding
--- PASS: TestAccBranding (8.76s)
PASS
coverage: 7.6% of statements
ok      github.com/alexkappa/terraform-provider-auth0/auth0     9.120s  coverage: 7.6% of statements
?       github.com/alexkappa/terraform-provider-auth0/auth0/internal/debug      [no test files]
testing: warning: no tests to run
PASS
coverage: 0.0% of statements
ok      github.com/alexkappa/terraform-provider-auth0/auth0/internal/random     0.423s  coverage: 0.0% of statements [no tests to run]
testing: warning: no tests to run
PASS
coverage: 0.0% of statements
ok      github.com/alexkappa/terraform-provider-auth0/auth0/internal/validation 0.478s  coverage: 0.0% of statements [no tests to run]
?       github.com/alexkappa/terraform-provider-auth0/version   [no test files]

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[nekootoko3]

Testing in an environment with no custom domain is complete.
But I could not test in an enviroment with custom domain because I don't have any paid enviroment which I can test in.

[nekootoko3]

in the case of t.Flags.EnableCustomDomainInEmails == true and UniversalLogin is not set, the response is 404 not found and branding cannot be updated.

[nekootoko3]

@alexkappa @mpalmer To merge this PR, is there anything for me to do?

[sergiught]

Hey @nekootoko3! Thanks a lot for this great contribution!

I took a look at it and pushed some small adjustments (mostly styling) and rebased on top of master.

This was a really useful fix!