Fix several issues on Datadog integration service form

What does this MR do?

Fixes form validation issues we found with the first user to test the integration. The issues are:

• Adding blank query parameters env and service to the URL
• Accepting blank string on the :datadog_site field
• Wrong validation of :datadog_site and :api_url together when both are blank strings
• Default value for :datadog_site not working as intended.
• Test button hangs when testing before saving, with invalid data.

Datadog integration discussion in #270123

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team