fix(ls): fix linting of OpenAPI 3 Server Variable enum field

[char0n]

OpenAPI 3.0.x - enum SHOULD NOT be empty
OpenAPI 3.1.0 - enum MUST NOT be empty

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For OpenAPI 3.0.x, empty enum field produces Warning. For OpenAPI 3.1.0, empty enum field produces Error.

Diference between Warning and Error:

Error - Something not allowed by the rules of a language or other means.
Warning - Something suspicious but allowed.