Fixed enum values processing for console operations' query and template parameters

[jsorohova]

Problem: If you have a query parameter where you set "No default" and provide other optional values, the "Try It" sidebar defaults to the next possible value under "No default", rather than starting as blank (without a default) as expected.

Solution: Added a placeholder when no default value is selected. Added a validation message when a parameter is not required.

Closes: #1529, #1730, #2384

[github-actions[bot]]

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